diff --git a/.github/workflows/test-upstream-wpt-changes.yml b/.github/workflows/test-upstream-wpt-changes.yml
new file mode 100644
index 00000000000..4eb006829b9
--- /dev/null
+++ b/.github/workflows/test-upstream-wpt-changes.yml
@@ -0,0 +1,18 @@
+name: Test changes to upstream-wpt-changes
+on:
+  pull_request:
+    branches: ["**"]
+    paths: ["etc/ci/upstream-wpt-changes/**"]
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install dependencies
+        run: |
+          python3 -m pip install --upgrade -r etc/ci/upstream-wpt-changes/requirements-dev.txt
+      - name: Running tests
+        run: |
+          python3 etc/ci/upstream-wpt-changes/test.py
diff --git a/.github/workflows/upstream-wpt-changes.yml b/.github/workflows/upstream-wpt-changes.yml
new file mode 100644
index 00000000000..c03fd8f0b35
--- /dev/null
+++ b/.github/workflows/upstream-wpt-changes.yml
@@ -0,0 +1,34 @@
+# Disabled until the previous bot is turned off.
+#name: Upstream Web Platform Test changes
+#on:
+#  pull_request:
+#    types: ['opened', 'synchronize', 'reopened', 'edited', 'closed']
+#
+#jobs:
+#  upstream:
+#    runs-on: ubuntu-latest
+#    steps:
+#      - name: Calculate PR fetch depth
+#        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+#      - name: Check out shallow servo PR
+#        run: |
+#          mkdir servo
+#          cd servo
+#          git init -b main
+#          git remote add origin ${{ github.event.repository.clone_url}}
+#          git fetch origin pull/${{ github.event.pull_request.number}}/head:pr --depth ${{ env.PR_FETCH_DEPTH }}
+#          git checkout pr
+#      - name: Check out wpt
+#        uses: actions/checkout@v3
+#        with:
+#          path: wpt
+#          repository: 'web-platform-tests/wpt'
+#          token: ${{ secrets.WPT_UPSTREAM_TOKEN }}
+#      - name: Install requirements
+#        run: pip install -r servo/etc/ci/upstream-wpt-changes/requirements.txt
+#      - name: Process pull request
+#        run: servo/etc/ci/upstream-wpt-changes/upstream-wpt-changes.py
+#        env:
+#          GITHUB_CONTEXT: ${{ toJson(github) }}
+#          GITHUB_TOKEN: ${{ secrets.WPT_UPSTREAM_TOKEN }}
+#
diff --git a/etc/ci/upstream-wpt-changes/README b/etc/ci/upstream-wpt-changes/README
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/etc/ci/upstream-wpt-changes/requirements-dev.txt b/etc/ci/upstream-wpt-changes/requirements-dev.txt
new file mode 100644
index 00000000000..d60a3d3dad9
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/requirements-dev.txt
@@ -0,0 +1,4 @@
+-r requirements.txt
+
+flask
+types-requests
\ No newline at end of file
diff --git a/etc/ci/upstream-wpt-changes/requirements.txt b/etc/ci/upstream-wpt-changes/requirements.txt
new file mode 100644
index 00000000000..663bd1f6a2a
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/requirements.txt
@@ -0,0 +1 @@
+requests
\ No newline at end of file
diff --git a/etc/ci/upstream-wpt-changes/test.py b/etc/ci/upstream-wpt-changes/test.py
new file mode 100644
index 00000000000..67d83603805
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/test.py
@@ -0,0 +1,621 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+# pylint: disable=broad-except
+# pylint: disable=dangerous-default-value
+# pylint: disable=global-statement
+# pylint: disable=line-too-long
+# pylint: disable=missing-docstring
+# pylint: disable=protected-access
+
+# This allows using types that are defined later in the file.
+from __future__ import annotations
+
+import dataclasses
+import json
+import locale
+import logging
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import threading
+import time
+import unittest
+
+from functools import partial
+from typing import Any, Optional, Tuple
+from wsgiref.simple_server import WSGIRequestHandler, make_server
+
+import flask
+import flask.cli
+import requests
+from wptupstreamer import SyncRun, WPTSync
+from wptupstreamer.step import CreateOrUpdateBranchForPRStep
+
+TESTS_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "tests")
+SYNC: Optional[WPTSync] = None
+TMP_DIR: Optional[str] = None
+PORT = 9000
+
+if "-v" in sys.argv or "--verbose" in sys.argv:
+    logging.getLogger().level = logging.DEBUG
+
+
+@dataclasses.dataclass
+class MockPullRequest():
+    head: str
+    number: int
+    state: str = "open"
+
+
+class MockGitHubAPIServer():
+    def __init__(self, port: int):
+        self.port = port
+        self.disable_logging()
+        self.app = flask.Flask(__name__)
+        self.pulls: list[MockPullRequest] = []
+
+        class NoLoggingHandler(WSGIRequestHandler):
+            def log_message(self, *args):
+                pass
+        if logging.getLogger().level == logging.DEBUG:
+            handler = WSGIRequestHandler
+        else:
+            handler = NoLoggingHandler
+
+        self.server = make_server('localhost', self.port, self.app, handler_class=handler)
+        self.start_server_thread()
+
+    def disable_logging(self):
+        flask.cli.show_server_banner = lambda *args: None
+        logging.getLogger("werkzeug").disabled = True
+        logging.getLogger('werkzeug').setLevel(logging.CRITICAL)
+
+    def start(self):
+        self.thread.start()
+
+        # Wait for the server to be started.
+        while True:
+            try:
+                response = requests.get(f'http://localhost:{self.port}/ping', timeout=1)
+                assert response.status_code == 200
+                assert response.text == 'pong'
+                break
+            except Exception:
+                time.sleep(0.1)
+
+    def reset_server_state_with_pull_requests(self, pulls: list[MockPullRequest]):
+        response = requests.get(
+            f'http://localhost:{self.port}/reset-mock-github',
+            json=[dataclasses.asdict(pull_request) for pull_request in pulls],
+            timeout=1
+        )
+        assert response.status_code == 200
+        assert response.text == '👍'
+
+    def shutdown(self):
+        self.server.shutdown()
+        self.thread.join()
+
+    def start_server_thread(self):
+        # pylint: disable=unused-argument
+        self.thread = threading.Thread(target=self.server.serve_forever, daemon=True)
+        self.thread.start()
+
+        @self.app.route("/ping")
+        def ping():
+            return ('pong', 200)
+
+        @self.app.route("/reset-mock-github")
+        def reset_server():
+            self.pulls = [
+                MockPullRequest(pull_request['head'],
+                                pull_request['number'],
+                                pull_request['state'])
+                for pull_request in flask.request.json]
+            return ('👍', 200)
+
+        @self.app.route("/repos/<org>/<repo>/pulls/<int:number>/merge", methods=['PUT'])
+        def merge_pull_request(org, repo, number):
+            for pull_request in self.pulls:
+                if pull_request.number == number:
+                    pull_request.state = 'closed'
+                    return ('', 204)
+            return ('', 404)
+
+        @self.app.route("/repos/<org>/<repo>/pulls", methods=['GET'])
+        def get_pulls(org, repo):
+            for pull_request in self.pulls:
+                if pull_request.head == flask.request.args["head"]:
+                    return json.dumps([{"number": pull_request.number}])
+            return json.dumps([])
+
+        @self.app.route("/repos/<org>/<repo>/pulls", methods=['POST'])
+        def create_pull_request(org, repo):
+            new_pr_number = len(self.pulls) + 1
+            self.pulls.append(MockPullRequest(
+                flask.request.json["head"],
+                new_pr_number,
+                "open"
+            ))
+            return {"number": new_pr_number}
+
+        @self.app.route("/repos/<org>/<repo>/pulls/<int:number>", methods=['PATCH'])
+        def update_pull_request(org, repo, number):
+            for pull_request in self.pulls:
+                if pull_request.number == number:
+                    if 'state' in flask.request.json:
+                        pull_request.state = flask.request.json['state']
+                    return ('', 204)
+            return ('', 404)
+
+        @self.app.route("/repos/<org>/<repo>/issues/<number>/labels", methods=['GET', 'POST'])
+        @self.app.route("/repos/<org>/<repo>/issues/<number>/labels/<label>", methods=['DELETE'])
+        @self.app.route("/repos/<org>/<repo>/issues/<issue>/comments", methods=['GET', 'POST'])
+        def other_requests(*args, **kwargs):
+            return ('', 204)
+
+
+class TestCleanUpBodyText(unittest.TestCase):
+    """Tests that SyncRun.clean_up_body_text properly prepares the
+    body text for an upstream pull request."""
+
+    def test_prepare_body(self):
+        text = "Simple body text"
+        self.assertEqual(text, SyncRun.clean_up_body_text(text))
+        self.assertEqual(
+            "With reference: #<!-- nolink -->3",
+            SyncRun.clean_up_body_text("With reference: #3"),
+        )
+        self.assertEqual(
+            "Multi reference: #<!-- nolink -->3 and #<!-- nolink -->1020",
+            SyncRun.clean_up_body_text("Multi reference: #3 and #1020"),
+        )
+        self.assertEqual(
+            "Subject\n\nBody text #<!-- nolink -->1",
+            SyncRun.clean_up_body_text(
+                "Subject\n\nBody text #1\n---<!-- Thank you for contributing"
+            ),
+        )
+        self.assertEqual(
+            "Subject\n\nNo dashes",
+            SyncRun.clean_up_body_text(
+                "Subject\n\nNo dashes<!-- Thank you for contributing"
+            ),
+        )
+        self.assertEqual(
+            "Subject\n\nNo --- comment",
+            SyncRun.clean_up_body_text(
+                "Subject\n\nNo --- comment\n---Other stuff that"
+            ),
+        )
+        self.assertEqual(
+            "Subject\n\n#<!-- nolink -->3 servo#<!-- nolink -->3 servo/servo#3",
+            SyncRun.clean_up_body_text(
+                "Subject\n\n#3 servo#3 servo/servo#3",
+            ),
+            "Only relative and bare issue reference links should be escaped."
+        )
+
+
+class TestApplyCommitsToWPT(unittest.TestCase):
+    """Tests that commits are properly applied to WPT by
+    CreateOrUpdateBranchForPRStep._create_or_update_branch_for_pr."""
+
+    def run_test(self, pr_number: int, commit_data: dict):
+        def make_commit(data):
+            with open(os.path.join(TESTS_DIR, data[2]), encoding="utf-8") as file:
+                return {"author": data[0], "message": data[1], "diff": file.read()}
+
+        commits = [make_commit(data) for data in commit_data]
+
+        assert SYNC is not None
+        pull_request = SYNC.servo.get_pull_request(pr_number)
+        step = CreateOrUpdateBranchForPRStep({"number": pr_number}, pull_request)
+
+        def get_applied_commits(
+            num_commits: int, applied_commits: list[Tuple[str, str]]
+        ):
+            assert SYNC is not None
+            repo = SYNC.local_wpt_repo
+            log = ["log", "--oneline", f"-{num_commits}"]
+            applied_commits += list(
+                zip(
+                    repo.run(*log, "--format=%aN <%ae>").splitlines(),
+                    repo.run(*log, "--format=%s").splitlines(),
+                )
+            )
+            applied_commits.reverse()
+
+        applied_commits: list[Any] = []
+        callback = partial(get_applied_commits, len(commits), applied_commits)
+        step._create_or_update_branch_for_pr(
+            SyncRun(SYNC, pull_request, None, None), commits, callback
+        )
+
+        expected_commits = [(commit["author"], commit["message"]) for commit in commits]
+        self.assertListEqual(applied_commits, expected_commits)
+
+    def test_simple_commit(self):
+        self.run_test(
+            45, [["test author <test@author>", "test commit message", "18746.diff"]]
+        )
+
+    def test_two_commits(self):
+        self.run_test(
+            100,
+            [
+                ["test author <test@author>", "test commit message", "18746.diff"],
+                ["another person <two@author>", "a different message", "wpt.diff"],
+            ],
+        )
+
+
+class TestFullSyncRun(unittest.TestCase):
+    server: Optional[MockGitHubAPIServer] = None
+
+    @classmethod
+    def setUpClass(cls):
+        cls.server = MockGitHubAPIServer(PORT)
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.server.shutdown()
+
+    def tearDown(self):
+        # Clean up any old files.
+        first_commit_hash = SYNC.local_servo_repo.run("rev-list", "HEAD").splitlines()[
+            -1
+        ]
+        SYNC.local_servo_repo.run("reset", "--hard", first_commit_hash)
+        SYNC.local_servo_repo.run("clean", "-fxd")
+
+    def mock_servo_repository_state(self, diffs: list):
+        assert SYNC is not None
+
+        def make_commit_data(diff):
+            if not isinstance(diff, list):
+                return [diff, "tmp author", "tmp@tmp.com", "tmp commit message"]
+            return diff
+
+        commits = [make_commit_data(diff) for diff in diffs]
+
+        # Apply each commit to the repository.
+        for commit in commits:
+            patch_file, author, email, message = commit
+            SYNC.local_servo_repo.run("apply", os.path.join(TESTS_DIR, patch_file))
+            SYNC.local_servo_repo.run("add", ".")
+            SYNC.local_servo_repo.run(
+                "commit",
+                "-a",
+                "--author",
+                f"{author} <{email}>",
+                "-m",
+                message,
+                env={
+                    "GIT_COMMITTER_NAME": author.encode(locale.getpreferredencoding()),
+                    "GIT_COMMITTER_EMAIL": email,
+                },
+            )
+
+    def run_test(
+        self, payload_file: str, diffs: list, existing_prs: list[MockPullRequest] = []
+    ):
+        with open(os.path.join(TESTS_DIR, payload_file), encoding="utf-8") as file:
+            payload = json.loads(file.read())
+
+        logging.info("Mocking application of PR to servo.")
+        self.mock_servo_repository_state(diffs)
+
+        logging.info("Resetting server state")
+        assert self.server is not None
+        self.server.reset_server_state_with_pull_requests(existing_prs)
+
+        actual_steps = []
+        assert SYNC is not None
+        SYNC.run(payload, step_callback=lambda step: actual_steps.append(step.name))
+        return actual_steps
+
+    def test_opened_upstreamable_pr(self):
+        self.assertListEqual(
+            self.run_test("opened.json", ["18746.diff"]),
+            [
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_18746",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_18746→wpt/wpt#1",
+                "CommentStep:servo/servo#18746:🤖 Opened new upstream WPT pull request "
+                "(wpt/wpt#1) with upstreamable changes.",
+            ],
+        )
+
+    def test_opened_upstreamable_pr_with_move_into_wpt(self):
+        self.assertListEqual(
+            self.run_test("opened.json", ["move-into-wpt.diff"]),
+            [
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_18746",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_18746→wpt/wpt#1",
+                "CommentStep:servo/servo#18746:🤖 Opened new upstream WPT pull request "
+                "(wpt/wpt#1) with upstreamable changes.",
+            ],
+        )
+
+    def test_opened_upstreamble_pr_with_move_into_wpt_and_non_ascii_author(self):
+        self.assertListEqual(
+            self.run_test(
+                "opened.json",
+                [
+                    [
+                        "move-into-wpt.diff",
+                        "Fernando Jiménez Moreno",
+                        "foo@bar.com",
+                        "ééééé",
+                    ]
+                ],
+            ),
+            [
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_18746",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_18746→wpt/wpt#1",
+                "CommentStep:servo/servo#18746:🤖 Opened new upstream WPT pull request "
+                "(wpt/wpt#1) with upstreamable changes.",
+            ],
+        )
+
+    def test_opened_upstreamable_pr_with_move_out_of_wpt(self):
+        self.assertListEqual(
+            self.run_test("opened.json", ["move-out-of-wpt.diff"]),
+            [
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_18746",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_18746→wpt/wpt#1",
+                "CommentStep:servo/servo#18746:🤖 Opened new upstream WPT pull request "
+                "(wpt/wpt#1) with upstreamable changes.",
+            ],
+        )
+
+    def test_opened_new_mr_with_no_sync_signal(self):
+        self.assertListEqual(
+            self.run_test("opened-with-no-sync-signal.json", ["18746.diff"]), []
+        )
+        self.assertListEqual(
+            self.run_test("opened-with-no-sync-signal.json", ["non-wpt.diff"]), []
+        )
+
+    def test_opened_upstreamable_pr_not_applying_cleanly_to_upstream(self):
+        self.assertListEqual(
+            self.run_test("opened.json", ["does-not-apply-cleanly.diff"]),
+            [
+                "CreateOrUpdateBranchForPRStep",
+                "CommentStep:servo/servo#18746:🛠 These changes could not be applied "
+                "onto the latest upstream WPT. Servo's copy of the Web Platform Tests may be out of sync.",
+            ],
+        )
+
+    def test_open_new_upstreamable_pr_with_preexisting_upstream_pr(self):
+        self.assertListEqual(
+            self.run_test(
+                "opened.json",
+                ["18746.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_18746", 1)],
+            ),
+            [
+                "ChangePRStep:wpt/wpt#1:opened:This is a test:<!-- Please...[95]",
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_18746",
+                "CommentStep:servo/servo#18746:📝 Transplanted new upstreamable changes to "
+                "existing upstream WPT pull request (wpt/wpt#1).",
+            ],
+        )
+
+    def test_open_new_non_upstreamable_pr_with_preexisting_upstream_pr(self):
+        self.assertListEqual(
+            self.run_test(
+                "opened.json",
+                ["non-wpt.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_18746", 1)],
+            ),
+            [
+                "CommentStep:wpt/wpt#1:👋 Downstream pull request (servo/servo#18746) no longer "
+                "contains any upstreamable changes. Closing pull request without merging.",
+                "ChangePRStep:wpt/wpt#1:closed",
+                "RemoveBranchForPRStep:servo-wpt-sync/wpt/servo_export_18746",
+                "CommentStep:servo/servo#18746:🤖 This change no longer contains upstreamable changes "
+                "to WPT; closed existing upstream pull request (wpt/wpt#1).",
+            ]
+        )
+
+    def test_open_new_upstreamable_pr_with_preexisting_upstream_pr_not_apply_cleanly_to_upstream(
+        self,
+    ):
+        self.assertListEqual(
+            self.run_test(
+                "opened.json",
+                ["does-not-apply-cleanly.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_18746", 1)],
+            ),
+            [
+                "ChangePRStep:wpt/wpt#1:opened:This is a test:<!-- Please...[95]",
+                "CreateOrUpdateBranchForPRStep",
+                "CommentStep:servo/servo#18746:🛠 These changes could not be applied onto the latest "
+                "upstream WPT. Servo's copy of the Web Platform Tests may be out of sync.",
+                "CommentStep:wpt/wpt#1:🛠 Changes from the source pull request (servo/servo#18746) can "
+                "no longer be cleanly applied. Waiting for a new version of these changes downstream.",
+            ],
+        )
+
+    def test_closed_pr_no_upstream_pr(self):
+        self.assertListEqual(self.run_test("closed.json", ["18746.diff"]), [])
+
+    def test_closed_pr_with_preexisting_upstream_pr(self):
+        self.assertListEqual(
+            self.run_test(
+                "closed.json",
+                ["18746.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_18746", 10)],
+            ),
+            [
+                "ChangePRStep:wpt/wpt#10:closed",
+                "RemoveBranchForPRStep:servo-wpt-sync/wpt/servo_export_18746"
+            ]
+        )
+
+    def test_synchronize_move_new_changes_to_preexisting_upstream_pr(self):
+        self.assertListEqual(
+            self.run_test(
+                "synchronize.json",
+                ["18746.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_19612", 10)],
+            ),
+            [
+                "ChangePRStep:wpt/wpt#10:opened:deny warnings:<!-- Please...[142]",
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_19612",
+                "CommentStep:servo/servo#19612:📝 Transplanted new upstreamable changes to existing "
+                "upstream WPT pull request (wpt/wpt#10).",
+            ]
+        )
+
+    def test_synchronize_close_upstream_pr_after_new_changes_do_not_include_wpt(self):
+        self.assertListEqual(
+            self.run_test(
+                "synchronize.json",
+                ["non-wpt.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_19612", 11)],
+            ),
+            [
+                "CommentStep:wpt/wpt#11:👋 Downstream pull request (servo/servo#19612) no longer contains any "
+                "upstreamable changes. Closing pull request without merging.",
+                "ChangePRStep:wpt/wpt#11:closed",
+                "RemoveBranchForPRStep:servo-wpt-sync/wpt/servo_export_19612",
+                "CommentStep:servo/servo#19612:🤖 This change no longer contains upstreamable changes to WPT; "
+                "closed existing upstream pull request (wpt/wpt#11).",
+            ]
+        )
+
+    def test_synchronize_open_upstream_pr_after_new_changes_include_wpt(self):
+        self.assertListEqual(
+            self.run_test("synchronize.json", ["18746.diff"]),
+            [
+                "CreateOrUpdateBranchForPRStep:1:servo-wpt-sync/wpt/servo_export_19612",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_19612→wpt/wpt#1",
+                "CommentStep:servo/servo#19612:🤖 Opened new upstream WPT pull request "
+                "(wpt/wpt#1) with upstreamable changes.",
+            ]
+        )
+
+    def test_synchronize_fail_to_update_preexisting_pr_after_new_changes_do_not_apply(
+        self,
+    ):
+        self.assertListEqual(
+            self.run_test(
+                "synchronize.json",
+                ["does-not-apply-cleanly.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_19612", 11)],
+            ),
+            [
+                "ChangePRStep:wpt/wpt#11:opened:deny warnings:<!-- Please...[142]",
+                "CreateOrUpdateBranchForPRStep",
+                "CommentStep:servo/servo#19612:🛠 These changes could not be applied onto the "
+                "latest upstream WPT. Servo's copy of the Web Platform Tests may be out of sync.",
+                "CommentStep:wpt/wpt#11:🛠 Changes from the source pull request (servo/servo#19612) can "
+                "no longer be cleanly applied. Waiting for a new version of these changes downstream.",
+            ]
+        )
+
+    def test_edited_with_upstream_pr(self):
+        self.assertListEqual(
+            self.run_test(
+                "edited.json", ["wpt.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_19620", 10)]
+            ),
+            [
+                "ChangePRStep:wpt/wpt#10:open:A cool new title:Reference #<!--...[136]",
+                "CommentStep:servo/servo#19620:✍ Updated existing upstream WPT pull "
+                "request (wpt/wpt#10) title and body."
+            ]
+        )
+
+    def test_edited_with_no_upstream_pr(self):
+        self.assertListEqual(self.run_test("edited.json", ["wpt.diff"], []), [])
+
+    def test_synchronize_move_new_changes_to_preexisting_upstream_pr_with_multiple_commits(
+        self,
+    ):
+        self.assertListEqual(
+            self.run_test(
+                "synchronize-multiple.json", ["18746.diff", "non-wpt.diff", "wpt.diff"]
+            ),
+            [
+                "CreateOrUpdateBranchForPRStep:2:servo-wpt-sync/wpt/servo_export_19612",
+                "OpenPRStep:servo-wpt-sync/wpt/servo_export_19612→wpt/wpt#1",
+                "CommentStep:servo/servo#19612:"
+                "🤖 Opened new upstream WPT pull request (wpt/wpt#1) with upstreamable changes.",
+            ]
+        )
+
+    def test_synchronize_with_non_upstreamable_changes(self):
+        self.assertListEqual(self.run_test("synchronize.json", ["non-wpt.diff"]), [])
+
+    def test_merge_upstream_pr_after_merge(self):
+        self.assertListEqual(
+            self.run_test(
+                "merged.json",
+                ["18746.diff"],
+                [MockPullRequest("servo-wpt-sync:servo_export_19620", 100)]
+            ),
+            [
+                "MergePRStep:wpt/wpt#100",
+                "RemoveBranchForPRStep:servo-wpt-sync/wpt/servo_export_19620"
+            ]
+        )
+
+    def test_pr_merged_no_upstream_pr(self):
+        self.assertListEqual(self.run_test("merged.json", ["18746.diff"]), [])
+
+    def test_merge_of_non_upstreamble_pr(self):
+        self.assertListEqual(self.run_test("merged.json", ["non-wpt.diff"]), [])
+
+
+def setUpModule():
+    # pylint: disable=invalid-name
+    global TMP_DIR, SYNC
+
+    TMP_DIR = tempfile.mkdtemp()
+    SYNC = WPTSync(
+        servo_repo="servo/servo",
+        wpt_repo="wpt/wpt",
+        downstream_wpt_repo="servo-wpt-sync/wpt",
+        servo_path=os.path.join(TMP_DIR, "servo-mock"),
+        wpt_path=os.path.join(TMP_DIR, "wpt-mock"),
+        github_api_token="",
+        github_api_url=f"http://localhost:{PORT}",
+        github_username="servo-wpt-sync",
+        github_email="servo-wpt-sync",
+        github_name="servo-wpt-sync@servo.org",
+        suppress_force_push=True,
+    )
+
+    def setup_mock_repo(repo_name, local_repo):
+        subprocess.check_output(
+            ["cp", "-R", "-p", os.path.join(TESTS_DIR, repo_name), local_repo.path])
+        local_repo.run("init", "-b", "master")
+        local_repo.run("add", ".")
+        local_repo.run("commit", "-a", "-m", "Initial commit")
+
+    logging.info("=" * 80)
+    logging.info("Setting up mock repositories")
+    setup_mock_repo("servo-mock", SYNC.local_servo_repo)
+    setup_mock_repo("wpt-mock", SYNC.local_wpt_repo)
+    logging.info("=" * 80)
+
+
+def tearDownModule():
+    # pylint: disable=invalid-name
+    shutil.rmtree(TMP_DIR)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/etc/ci/upstream-wpt-changes/tests/18746.diff b/etc/ci/upstream-wpt-changes/tests/18746.diff
new file mode 100644
index 00000000000..ad6150c1bdb
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/18746.diff
@@ -0,0 +1,10 @@
+diff --git a/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html b/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html
+index ac35dea54c47..135ad21d15ab 100644
+--- a/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html
++++ b/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html
+@@ -13,4 +13,5 @@
+     <script src="redirect-location.js"></script>
+   </body>
+ </html>
++test commit
+
diff --git a/etc/ci/upstream-wpt-changes/tests/closed.json b/etc/ci/upstream-wpt-changes/tests/closed.json
new file mode 100644
index 00000000000..e81ff80eb2d
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/closed.json
@@ -0,0 +1,476 @@
+{
+  "action": "closed",
+  "number": 18746,
+  "pull_request": {
+    "url": "https://api.github.com/repos/servo/servo/pulls/18746",
+    "id": 144666315,
+    "html_url": "https://github.com/servo/servo/pull/18746",
+    "diff_url": "https://github.com/servo/servo/pull/18746.diff",
+    "patch_url": "https://github.com/servo/servo/pull/18746.patch",
+    "issue_url": "https://api.github.com/repos/servo/servo/issues/18746",
+    "number": 18746,
+    "state": "closed",
+    "locked": false,
+    "title": "This is a test",
+    "user": {
+      "login": "jdm",
+      "id": 27658,
+      "avatar_url": "https://avatars1.githubusercontent.com/u/27658?v=4",
+      "gravatar_id": "",
+      "url": "https://api.github.com/users/jdm",
+      "html_url": "https://github.com/jdm",
+      "followers_url": "https://api.github.com/users/jdm/followers",
+      "following_url": "https://api.github.com/users/jdm/following{/other_user}",
+      "gists_url": "https://api.github.com/users/jdm/gists{/gist_id}",
+      "starred_url": "https://api.github.com/users/jdm/starred{/owner}{/repo}",
+      "subscriptions_url": "https://api.github.com/users/jdm/subscriptions",
+      "organizations_url": "https://api.github.com/users/jdm/orgs",
+      "repos_url": "https://api.github.com/users/jdm/repos",
+      "events_url": "https://api.github.com/users/jdm/events{/privacy}",
+      "received_events_url": "https://api.github.com/users/jdm/received_events",
+      "type": "User",
+      "site_admin": false
+    },
+    "body": "<!-- Please describe your changes on the following line: -->\r\n\r\n\r\n---\r\n<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->\r\n- [ ] `./mach build -d` does not report any errors\r\n- [ ] `./mach test-tidy` does not report any errors\r\n- [ ] These changes fix #__ (github issue number if applicable).\r\n\r\n<!-- Either: -->\r\n- [ ] There are tests for these changes OR\r\n- [ ] These changes do not require tests because _____\r\n\r\n<!-- Also, please make sure that \"Allow edits from maintainers\" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->\r\n\r\n<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->\r\n",
+    "created_at": "2017-10-04T13:16:26Z",
+    "updated_at": "2017-10-04T13:16:30Z",
+    "closed_at": "2017-10-04T13:16:30Z",
+    "merged_at": null,
+    "merge_commit_sha": "04b78ff84073a6b6e2126fea39c8878f3f7af72b",
+    "assignee": {
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+      "received_events_url": "https://api.github.com/users/emilio/received_events",
+      "type": "User",
+      "site_admin": false
+    },
+    "assignees": [
+      {
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+        "avatar_url": "https://avatars1.githubusercontent.com/u/1323194?v=4",
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+        "url": "https://api.github.com/users/emilio",
+        "html_url": "https://github.com/emilio",
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+        "received_events_url": "https://api.github.com/users/emilio/received_events",
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+
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+    "head": {
+      "label": "servo:jdm-patch-1",
+      "ref": "jdm-patch-1",
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+        "gravatar_id": "",
+        "url": "https://api.github.com/users/servo",
+        "html_url": "https://github.com/servo",
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+        "organizations_url": "https://api.github.com/users/servo/orgs",
+        "repos_url": "https://api.github.com/users/servo/repos",
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+          "type": "Organization",
+          "site_admin": false
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+        "private": false,
+        "html_url": "https://github.com/servo/servo",
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+        "fork": false,
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+        "merges_url": "https://api.github.com/repos/servo/servo/merges",
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+        "ssh_url": "git@github.com:servo/servo.git",
+        "clone_url": "https://github.com/servo/servo.git",
+        "svn_url": "https://github.com/servo/servo",
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index 00000000000..aa49c3bafbe
--- /dev/null
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+index cffb6eb..6100a49 100644
+--- a/tests/wpt/web-platform-tests/css/out-of-sync-test.html
++++ b/tests/wpt/web-platform-tests/css/out-of-sync-test.html
+@@ -1,3 +1,3 @@
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+-<h1>css test!</h1>
++<h1>css test!</h1>hi
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index 00000000000..4fe373ad7b3
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+      "site_admin": false
+    },
+    "private": false,
+    "html_url": "https://github.com/servo/servo",
+    "description": "The Servo Browser Engine",
+    "fork": false,
+    "url": "https://api.github.com/repos/servo/servo",
+    "forks_url": "https://api.github.com/repos/servo/servo/forks",
+    "keys_url": "https://api.github.com/repos/servo/servo/keys{/key_id}",
+    "collaborators_url": "https://api.github.com/repos/servo/servo/collaborators{/collaborator}",
+    "teams_url": "https://api.github.com/repos/servo/servo/teams",
+    "hooks_url": "https://api.github.com/repos/servo/servo/hooks",
+    "issue_events_url": "https://api.github.com/repos/servo/servo/issues/events{/number}",
+    "events_url": "https://api.github.com/repos/servo/servo/events",
+    "assignees_url": "https://api.github.com/repos/servo/servo/assignees{/user}",
+    "branches_url": "https://api.github.com/repos/servo/servo/branches{/branch}",
+    "tags_url": "https://api.github.com/repos/servo/servo/tags",
+    "blobs_url": "https://api.github.com/repos/servo/servo/git/blobs{/sha}",
+    "git_tags_url": "https://api.github.com/repos/servo/servo/git/tags{/sha}",
+    "git_refs_url": "https://api.github.com/repos/servo/servo/git/refs{/sha}",
+    "trees_url": "https://api.github.com/repos/servo/servo/git/trees{/sha}",
+    "statuses_url": "https://api.github.com/repos/servo/servo/statuses/{sha}",
+    "languages_url": "https://api.github.com/repos/servo/servo/languages",
+    "stargazers_url": "https://api.github.com/repos/servo/servo/stargazers",
+    "contributors_url": "https://api.github.com/repos/servo/servo/contributors",
+    "subscribers_url": "https://api.github.com/repos/servo/servo/subscribers",
+    "subscription_url": "https://api.github.com/repos/servo/servo/subscription",
+    "commits_url": "https://api.github.com/repos/servo/servo/commits{/sha}",
+    "git_commits_url": "https://api.github.com/repos/servo/servo/git/commits{/sha}",
+    "comments_url": "https://api.github.com/repos/servo/servo/comments{/number}",
+    "issue_comment_url": "https://api.github.com/repos/servo/servo/issues/comments{/number}",
+    "contents_url": "https://api.github.com/repos/servo/servo/contents/{+path}",
+    "compare_url": "https://api.github.com/repos/servo/servo/compare/{base}...{head}",
+    "merges_url": "https://api.github.com/repos/servo/servo/merges",
+    "archive_url": "https://api.github.com/repos/servo/servo/{archive_format}{/ref}",
+    "downloads_url": "https://api.github.com/repos/servo/servo/downloads",
+    "issues_url": "https://api.github.com/repos/servo/servo/issues{/number}",
+    "pulls_url": "https://api.github.com/repos/servo/servo/pulls{/number}",
+    "milestones_url": "https://api.github.com/repos/servo/servo/milestones{/number}",
+    "notifications_url": "https://api.github.com/repos/servo/servo/notifications{?since,all,participating}",
+    "labels_url": "https://api.github.com/repos/servo/servo/labels{/name}",
+    "releases_url": "https://api.github.com/repos/servo/servo/releases{/id}",
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+    "updated_at": "2017-12-21T09:43:03Z",
+    "pushed_at": "2017-12-21T14:53:23Z",
+    "git_url": "git://github.com/servo/servo.git",
+    "ssh_url": "git@github.com:servo/servo.git",
+    "clone_url": "https://github.com/servo/servo.git",
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+    "has_wiki": true,
+    "has_pages": false,
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+    "license": {
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+      "name": "Mozilla Public License 2.0",
+      "spdx_id": "MPL-2.0",
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+    "forks": 1810,
+    "open_issues": 2027,
+    "watchers": 10867,
+    "default_branch": "master"
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+    "login": "servo",
+    "id": 2566135,
+    "url": "https://api.github.com/orgs/servo",
+    "repos_url": "https://api.github.com/orgs/servo/repos",
+    "events_url": "https://api.github.com/orgs/servo/events",
+    "hooks_url": "https://api.github.com/orgs/servo/hooks",
+    "issues_url": "https://api.github.com/orgs/servo/issues",
+    "members_url": "https://api.github.com/orgs/servo/members{/member}",
+    "public_members_url": "https://api.github.com/orgs/servo/public_members{/member}",
+    "avatar_url": "https://avatars1.githubusercontent.com/u/2566135?v=4",
+    "description": "The Servo web browser engine"
+  },
+  "sender": {
+    "login": "bors-servo",
+    "id": 4368172,
+    "avatar_url": "https://avatars1.githubusercontent.com/u/4368172?v=4",
+    "gravatar_id": "",
+    "url": "https://api.github.com/users/bors-servo",
+    "html_url": "https://github.com/bors-servo",
+    "followers_url": "https://api.github.com/users/bors-servo/followers",
+    "following_url": "https://api.github.com/users/bors-servo/following{/other_user}",
+    "gists_url": "https://api.github.com/users/bors-servo/gists{/gist_id}",
+    "starred_url": "https://api.github.com/users/bors-servo/starred{/owner}{/repo}",
+    "subscriptions_url": "https://api.github.com/users/bors-servo/subscriptions",
+    "organizations_url": "https://api.github.com/users/bors-servo/orgs",
+    "repos_url": "https://api.github.com/users/bors-servo/repos",
+    "events_url": "https://api.github.com/users/bors-servo/events{/privacy}",
+    "received_events_url": "https://api.github.com/users/bors-servo/received_events",
+    "type": "User",
+    "site_admin": false
+  }
+}
diff --git a/etc/ci/upstream-wpt-changes/tests/move-into-wpt.diff b/etc/ci/upstream-wpt-changes/tests/move-into-wpt.diff
new file mode 100644
index 00000000000..02c437cdd7b
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/move-into-wpt.diff
@@ -0,0 +1,4 @@
+diff --git a/tests/wpt/mozilla/tests/mozilla/mozilla-test.html b/tests/wpt/web-platform-tests/mozilla-test.html
+similarity index 100%
+rename from tests/wpt/mozilla/tests/mozilla/mozilla-test.html
+rename to tests/wpt/web-platform-tests/mozilla-test.html
diff --git a/etc/ci/upstream-wpt-changes/tests/move-out-of-wpt.diff b/etc/ci/upstream-wpt-changes/tests/move-out-of-wpt.diff
new file mode 100644
index 00000000000..f81541363ec
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/move-out-of-wpt.diff
@@ -0,0 +1,4 @@
+diff --git a/tests/wpt/web-platform-tests/test.html b/tests/wpt/test.html
+similarity index 100%
+rename from tests/wpt/web-platform-tests/test.html
+rename to tests/wpt/test.html
diff --git a/etc/ci/upstream-wpt-changes/tests/non-wpt.diff b/etc/ci/upstream-wpt-changes/tests/non-wpt.diff
new file mode 100644
index 00000000000..0e3862805e4
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/non-wpt.diff
@@ -0,0 +1,10 @@
+diff --git a/tests/wpt/something.py b/tests/wpt/something.py
+index 10d52a0..92fb89d 100644
+--- a/tests/wpt/something.py
++++ b/tests/wpt/something.py
+@@ -8,3 +8,4 @@
+ # except according to those terms.
+ 
+ print('this is a python file')
++print('this is a change')
+
diff --git a/etc/ci/upstream-wpt-changes/tests/opened-with-no-sync-signal.json b/etc/ci/upstream-wpt-changes/tests/opened-with-no-sync-signal.json
new file mode 100644
index 00000000000..1a1671632a4
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/opened-with-no-sync-signal.json
@@ -0,0 +1,440 @@
+{
+  "action": "opened",
+  "number": 18746,
+  "pull_request": {
+    "url": "https://api.github.com/repos/servo/servo/pulls/18746",
+    "id": 144666315,
+    "html_url": "https://github.com/servo/servo/pull/18746",
+    "diff_url": "https://github.com/servo/servo/pull/18746.diff",
+    "patch_url": "https://github.com/servo/servo/pull/18746.patch",
+    "issue_url": "https://api.github.com/repos/servo/servo/issues/18746",
+    "number": 18746,
+    "state": "open",
+    "locked": false,
+    "title": "This is a test",
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+      "html_url": "https://github.com/jdm",
+      "followers_url": "https://api.github.com/users/jdm/followers",
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+      "gists_url": "https://api.github.com/users/jdm/gists{/gist_id}",
+      "starred_url": "https://api.github.com/users/jdm/starred{/owner}{/repo}",
+      "subscriptions_url": "https://api.github.com/users/jdm/subscriptions",
+      "organizations_url": "https://api.github.com/users/jdm/orgs",
+      "repos_url": "https://api.github.com/users/jdm/repos",
+      "events_url": "https://api.github.com/users/jdm/events{/privacy}",
+      "received_events_url": "https://api.github.com/users/jdm/received_events",
+      "type": "User",
+      "site_admin": false
+    },
+    "body": "[no-wpt-sync]<!-- Please describe your changes on the following line: -->\r\n\r\n\r\n---\r\n<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->\r\n- [ ] `./mach build -d` does not report any errors\r\n- [ ] `./mach test-tidy` does not report any errors\r\n- [ ] These changes fix #__ (github issue number if applicable).\r\n\r\n<!-- Either: -->\r\n- [ ] There are tests for these changes OR\r\n- [ ] These changes do not require tests because _____\r\n\r\n<!-- Also, please make sure that \"Allow edits from maintainers\" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->\r\n\r\n<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->\r\n",
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+    "updated_at": "2017-10-04T13:16:26Z",
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+    "merged_at": null,
+    "merge_commit_sha": null,
+    "assignee": null,
+    "assignees": [
+
+    ],
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+
+    ],
+    "milestone": null,
+    "commits_url": "https://api.github.com/repos/servo/servo/pulls/18746/commits",
+    "review_comments_url": "https://api.github.com/repos/servo/servo/pulls/18746/comments",
+    "review_comment_url": "https://api.github.com/repos/servo/servo/pulls/comments{/number}",
+    "comments_url": "https://api.github.com/repos/servo/servo/issues/18746/comments",
+    "statuses_url": "https://api.github.com/repos/servo/servo/statuses/1bb1cd385c4d1928b4f60c4c15906f5318ca2f3f",
+    "head": {
+      "label": "servo:jdm-patch-1",
+      "ref": "jdm-patch-1",
+      "sha": "1bb1cd385c4d1928b4f60c4c15906f5318ca2f3f",
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+        "url": "https://api.github.com/users/servo",
+        "html_url": "https://github.com/servo",
+        "followers_url": "https://api.github.com/users/servo/followers",
+        "following_url": "https://api.github.com/users/servo/following{/other_user}",
+        "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+        "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+        "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+        "organizations_url": "https://api.github.com/users/servo/orgs",
+        "repos_url": "https://api.github.com/users/servo/repos",
+        "events_url": "https://api.github.com/users/servo/events{/privacy}",
+        "received_events_url": "https://api.github.com/users/servo/received_events",
+        "type": "Organization",
+        "site_admin": false
+      },
+      "repo": {
+        "id": 3390243,
+        "name": "servo",
+        "full_name": "servo/servo",
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+          "avatar_url": "https://avatars1.githubusercontent.com/u/2566135?v=4",
+          "gravatar_id": "",
+          "url": "https://api.github.com/users/servo",
+          "html_url": "https://github.com/servo",
+          "followers_url": "https://api.github.com/users/servo/followers",
+          "following_url": "https://api.github.com/users/servo/following{/other_user}",
+          "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+          "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+          "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+          "organizations_url": "https://api.github.com/users/servo/orgs",
+          "repos_url": "https://api.github.com/users/servo/repos",
+          "events_url": "https://api.github.com/users/servo/events{/privacy}",
+          "received_events_url": "https://api.github.com/users/servo/received_events",
+          "type": "Organization",
+          "site_admin": false
+        },
+        "private": false,
+        "html_url": "https://github.com/servo/servo",
+        "description": "The Servo Browser Engine",
+        "fork": false,
+        "url": "https://api.github.com/repos/servo/servo",
+        "forks_url": "https://api.github.com/repos/servo/servo/forks",
+        "keys_url": "https://api.github.com/repos/servo/servo/keys{/key_id}",
+        "collaborators_url": "https://api.github.com/repos/servo/servo/collaborators{/collaborator}",
+        "teams_url": "https://api.github.com/repos/servo/servo/teams",
+        "hooks_url": "https://api.github.com/repos/servo/servo/hooks",
+        "issue_events_url": "https://api.github.com/repos/servo/servo/issues/events{/number}",
+        "events_url": "https://api.github.com/repos/servo/servo/events",
+        "assignees_url": "https://api.github.com/repos/servo/servo/assignees{/user}",
+        "branches_url": "https://api.github.com/repos/servo/servo/branches{/branch}",
+        "tags_url": "https://api.github.com/repos/servo/servo/tags",
+        "blobs_url": "https://api.github.com/repos/servo/servo/git/blobs{/sha}",
+        "git_tags_url": "https://api.github.com/repos/servo/servo/git/tags{/sha}",
+        "git_refs_url": "https://api.github.com/repos/servo/servo/git/refs{/sha}",
+        "trees_url": "https://api.github.com/repos/servo/servo/git/trees{/sha}",
+        "statuses_url": "https://api.github.com/repos/servo/servo/statuses/{sha}",
+        "languages_url": "https://api.github.com/repos/servo/servo/languages",
+        "stargazers_url": "https://api.github.com/repos/servo/servo/stargazers",
+        "contributors_url": "https://api.github.com/repos/servo/servo/contributors",
+        "subscribers_url": "https://api.github.com/repos/servo/servo/subscribers",
+        "subscription_url": "https://api.github.com/repos/servo/servo/subscription",
+        "commits_url": "https://api.github.com/repos/servo/servo/commits{/sha}",
+        "git_commits_url": "https://api.github.com/repos/servo/servo/git/commits{/sha}",
+        "comments_url": "https://api.github.com/repos/servo/servo/comments{/number}",
+        "issue_comment_url": "https://api.github.com/repos/servo/servo/issues/comments{/number}",
+        "contents_url": "https://api.github.com/repos/servo/servo/contents/{+path}",
+        "compare_url": "https://api.github.com/repos/servo/servo/compare/{base}...{head}",
+        "merges_url": "https://api.github.com/repos/servo/servo/merges",
+        "archive_url": "https://api.github.com/repos/servo/servo/{archive_format}{/ref}",
+        "downloads_url": "https://api.github.com/repos/servo/servo/downloads",
+        "issues_url": "https://api.github.com/repos/servo/servo/issues{/number}",
+        "pulls_url": "https://api.github.com/repos/servo/servo/pulls{/number}",
+        "milestones_url": "https://api.github.com/repos/servo/servo/milestones{/number}",
+        "notifications_url": "https://api.github.com/repos/servo/servo/notifications{?since,all,participating}",
+        "labels_url": "https://api.github.com/repos/servo/servo/labels{/name}",
+        "releases_url": "https://api.github.com/repos/servo/servo/releases{/id}",
+        "deployments_url": "https://api.github.com/repos/servo/servo/deployments",
+        "created_at": "2012-02-08T19:07:25Z",
+        "updated_at": "2017-10-04T11:44:53Z",
+        "pushed_at": "2017-10-04T13:16:20Z",
+        "git_url": "git://github.com/servo/servo.git",
+        "ssh_url": "git@github.com:servo/servo.git",
+        "clone_url": "https://github.com/servo/servo.git",
+        "svn_url": "https://github.com/servo/servo",
+        "homepage": "https://servo.org/",
+        "size": 370069,
+        "stargazers_count": 10131,
+        "watchers_count": 10131,
+        "language": null,
+        "has_issues": true,
+        "has_projects": false,
+        "has_downloads": true,
+        "has_wiki": true,
+        "has_pages": false,
+        "forks_count": 1731,
+        "mirror_url": null,
+        "open_issues_count": 2073,
+        "forks": 1731,
+        "open_issues": 2073,
+        "watchers": 10131,
+        "default_branch": "master"
+      }
+    },
+    "base": {
+      "label": "servo:master",
+      "ref": "master",
+      "sha": "3b7a82b8c743bcbbefb6f25defd4cc132c25c348",
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+        "url": "https://api.github.com/users/servo",
+        "html_url": "https://github.com/servo",
+        "followers_url": "https://api.github.com/users/servo/followers",
+        "following_url": "https://api.github.com/users/servo/following{/other_user}",
+        "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+        "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+        "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+        "organizations_url": "https://api.github.com/users/servo/orgs",
+        "repos_url": "https://api.github.com/users/servo/repos",
+        "events_url": "https://api.github.com/users/servo/events{/privacy}",
+        "received_events_url": "https://api.github.com/users/servo/received_events",
+        "type": "Organization",
+        "site_admin": false
+      },
+      "repo": {
+        "id": 3390243,
+        "name": "servo",
+        "full_name": "servo/servo",
+        "owner": {
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+          "avatar_url": "https://avatars1.githubusercontent.com/u/2566135?v=4",
+          "gravatar_id": "",
+          "url": "https://api.github.com/users/servo",
+          "html_url": "https://github.com/servo",
+          "followers_url": "https://api.github.com/users/servo/followers",
+          "following_url": "https://api.github.com/users/servo/following{/other_user}",
+          "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+          "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+          "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+          "organizations_url": "https://api.github.com/users/servo/orgs",
+          "repos_url": "https://api.github.com/users/servo/repos",
+          "events_url": "https://api.github.com/users/servo/events{/privacy}",
+          "received_events_url": "https://api.github.com/users/servo/received_events",
+          "type": "Organization",
+          "site_admin": false
+        },
+        "private": false,
+        "html_url": "https://github.com/servo/servo",
+        "description": "The Servo Browser Engine",
+        "fork": false,
+        "url": "https://api.github.com/repos/servo/servo",
+        "forks_url": "https://api.github.com/repos/servo/servo/forks",
+        "keys_url": "https://api.github.com/repos/servo/servo/keys{/key_id}",
+        "collaborators_url": "https://api.github.com/repos/servo/servo/collaborators{/collaborator}",
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diff --git a/etc/ci/upstream-wpt-changes/tests/opened.json b/etc/ci/upstream-wpt-changes/tests/opened.json
new file mode 100644
index 00000000000..97a02be58b5
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/opened.json
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+        "releases_url": "https://api.github.com/repos/servo/servo/releases{/id}",
+        "deployments_url": "https://api.github.com/repos/servo/servo/deployments",
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+        "updated_at": "2017-10-04T11:44:53Z",
+        "pushed_at": "2017-10-04T13:16:20Z",
+        "git_url": "git://github.com/servo/servo.git",
+        "ssh_url": "git@github.com:servo/servo.git",
+        "clone_url": "https://github.com/servo/servo.git",
+        "svn_url": "https://github.com/servo/servo",
+        "homepage": "https://servo.org/",
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+      "review_comments": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/18746/comments"
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+      "review_comment": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/comments{/number}"
+      },
+      "commits": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/18746/commits"
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+      "statuses": {
+        "href": "https://api.github.com/repos/servo/servo/statuses/1bb1cd385c4d1928b4f60c4c15906f5318ca2f3f"
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+    "rebaseable": null,
+    "mergeable_state": "unknown",
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+    "maintainer_can_modify": false,
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+    "additions": 2,
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+  "repository": {
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+    "name": "servo",
+    "full_name": "servo/servo",
+    "owner": {
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+      "gravatar_id": "",
+      "url": "https://api.github.com/users/servo",
+      "html_url": "https://github.com/servo",
+      "followers_url": "https://api.github.com/users/servo/followers",
+      "following_url": "https://api.github.com/users/servo/following{/other_user}",
+      "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+      "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+      "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+      "organizations_url": "https://api.github.com/users/servo/orgs",
+      "repos_url": "https://api.github.com/users/servo/repos",
+      "events_url": "https://api.github.com/users/servo/events{/privacy}",
+      "received_events_url": "https://api.github.com/users/servo/received_events",
+      "type": "Organization",
+      "site_admin": false
+    },
+    "private": false,
+    "html_url": "https://github.com/servo/servo",
+    "description": "The Servo Browser Engine",
+    "fork": false,
+    "url": "https://api.github.com/repos/servo/servo",
+    "forks_url": "https://api.github.com/repos/servo/servo/forks",
+    "keys_url": "https://api.github.com/repos/servo/servo/keys{/key_id}",
+    "collaborators_url": "https://api.github.com/repos/servo/servo/collaborators{/collaborator}",
+    "teams_url": "https://api.github.com/repos/servo/servo/teams",
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+    "events_url": "https://api.github.com/repos/servo/servo/events",
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+    "branches_url": "https://api.github.com/repos/servo/servo/branches{/branch}",
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+    "issue_comment_url": "https://api.github.com/repos/servo/servo/issues/comments{/number}",
+    "contents_url": "https://api.github.com/repos/servo/servo/contents/{+path}",
+    "compare_url": "https://api.github.com/repos/servo/servo/compare/{base}...{head}",
+    "merges_url": "https://api.github.com/repos/servo/servo/merges",
+    "archive_url": "https://api.github.com/repos/servo/servo/{archive_format}{/ref}",
+    "downloads_url": "https://api.github.com/repos/servo/servo/downloads",
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+    "followers_url": "https://api.github.com/users/jdm/followers",
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+    "subscriptions_url": "https://api.github.com/users/jdm/subscriptions",
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diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/README.md b/etc/ci/upstream-wpt-changes/tests/servo-mock/README.md
new file mode 100644
index 00000000000..c2e23e99864
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/README.md
@@ -0,0 +1 @@
+This is a repository designed to superficially resemble the Servo repository.
diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/mozilla/tests/mozilla/mozilla-test.html b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/mozilla/tests/mozilla/mozilla-test.html
new file mode 100644
index 00000000000..542e2924b96
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/mozilla/tests/mozilla/mozilla-test.html
@@ -0,0 +1,3 @@
+<html>
+<h1>this is a mozilla-specific test</h1>
+</html>
diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/something.py b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/something.py
new file mode 100644
index 00000000000..0f9ba39707a
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/something.py
@@ -0,0 +1,10 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+print('this is a python file')
diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/css/css-test.html b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/css/css-test.html
new file mode 100644
index 00000000000..cffb6eb6b5a
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/css/css-test.html
@@ -0,0 +1,3 @@
+<html>
+<h1>css test!</h1>
+</html>
diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/css/out-of-sync-test.html b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/css/out-of-sync-test.html
new file mode 100644
index 00000000000..cffb6eb6b5a
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/css/out-of-sync-test.html
@@ -0,0 +1,3 @@
+<html>
+<h1>css test!</h1>
+</html>
diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html
new file mode 100644
index 00000000000..48a68cd53ce
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/fetch/api/redirect/redirect-location.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Fetch: handling Location header during redirection</title>
+    <meta name="author" title="Canon Research France" href="https://www.crf.canon.fr">
+    <meta name="help" href="https://fetch.spec.whatwg.org/#http-network-or-cache-fetch">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+  </head>
+  <body>
+    <script src="../resources/utils.js"></script>
+    <script src="redirect-location.js"></script>
+  </body>
+</html>
+
diff --git a/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/test.html b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/test.html
new file mode 100644
index 00000000000..43b09d4e57d
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/servo-mock/tests/wpt/web-platform-tests/test.html
@@ -0,0 +1,3 @@
+<html>
+<h1>this is a test file</h1>
+</html>
diff --git a/etc/ci/upstream-wpt-changes/tests/synchronize-multiple.json b/etc/ci/upstream-wpt-changes/tests/synchronize-multiple.json
new file mode 100644
index 00000000000..239d8d00e5a
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/synchronize-multiple.json
@@ -0,0 +1,499 @@
+{
+  "action": "synchronize",
+  "number": 19612,
+  "pull_request": {
+    "url": "https://api.github.com/repos/servo/servo/pulls/19612",
+    "id": 159474132,
+    "html_url": "https://github.com/servo/servo/pull/19612",
+    "diff_url": "https://github.com/servo/servo/pull/19612.diff",
+    "patch_url": "https://github.com/servo/servo/pull/19612.patch",
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diff --git a/etc/ci/upstream-wpt-changes/tests/synchronize.json b/etc/ci/upstream-wpt-changes/tests/synchronize.json
new file mode 100644
index 00000000000..cbb0d8966b8
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/synchronize.json
@@ -0,0 +1,499 @@
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+        "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+        "organizations_url": "https://api.github.com/users/servo/orgs",
+        "repos_url": "https://api.github.com/users/servo/repos",
+        "events_url": "https://api.github.com/users/servo/events{/privacy}",
+        "received_events_url": "https://api.github.com/users/servo/received_events",
+        "type": "Organization",
+        "site_admin": false
+      },
+      "repo": {
+        "id": 3390243,
+        "name": "servo",
+        "full_name": "servo/servo",
+        "owner": {
+          "login": "servo",
+          "id": 2566135,
+          "avatar_url": "https://avatars1.githubusercontent.com/u/2566135?v=4",
+          "gravatar_id": "",
+          "url": "https://api.github.com/users/servo",
+          "html_url": "https://github.com/servo",
+          "followers_url": "https://api.github.com/users/servo/followers",
+          "following_url": "https://api.github.com/users/servo/following{/other_user}",
+          "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+          "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+          "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+          "organizations_url": "https://api.github.com/users/servo/orgs",
+          "repos_url": "https://api.github.com/users/servo/repos",
+          "events_url": "https://api.github.com/users/servo/events{/privacy}",
+          "received_events_url": "https://api.github.com/users/servo/received_events",
+          "type": "Organization",
+          "site_admin": false
+        },
+        "private": false,
+        "html_url": "https://github.com/servo/servo",
+        "description": "The Servo Browser Engine",
+        "fork": false,
+        "url": "https://api.github.com/repos/servo/servo",
+        "forks_url": "https://api.github.com/repos/servo/servo/forks",
+        "keys_url": "https://api.github.com/repos/servo/servo/keys{/key_id}",
+        "collaborators_url": "https://api.github.com/repos/servo/servo/collaborators{/collaborator}",
+        "teams_url": "https://api.github.com/repos/servo/servo/teams",
+        "hooks_url": "https://api.github.com/repos/servo/servo/hooks",
+        "issue_events_url": "https://api.github.com/repos/servo/servo/issues/events{/number}",
+        "events_url": "https://api.github.com/repos/servo/servo/events",
+        "assignees_url": "https://api.github.com/repos/servo/servo/assignees{/user}",
+        "branches_url": "https://api.github.com/repos/servo/servo/branches{/branch}",
+        "tags_url": "https://api.github.com/repos/servo/servo/tags",
+        "blobs_url": "https://api.github.com/repos/servo/servo/git/blobs{/sha}",
+        "git_tags_url": "https://api.github.com/repos/servo/servo/git/tags{/sha}",
+        "git_refs_url": "https://api.github.com/repos/servo/servo/git/refs{/sha}",
+        "trees_url": "https://api.github.com/repos/servo/servo/git/trees{/sha}",
+        "statuses_url": "https://api.github.com/repos/servo/servo/statuses/{sha}",
+        "languages_url": "https://api.github.com/repos/servo/servo/languages",
+        "stargazers_url": "https://api.github.com/repos/servo/servo/stargazers",
+        "contributors_url": "https://api.github.com/repos/servo/servo/contributors",
+        "subscribers_url": "https://api.github.com/repos/servo/servo/subscribers",
+        "subscription_url": "https://api.github.com/repos/servo/servo/subscription",
+        "commits_url": "https://api.github.com/repos/servo/servo/commits{/sha}",
+        "git_commits_url": "https://api.github.com/repos/servo/servo/git/commits{/sha}",
+        "comments_url": "https://api.github.com/repos/servo/servo/comments{/number}",
+        "issue_comment_url": "https://api.github.com/repos/servo/servo/issues/comments{/number}",
+        "contents_url": "https://api.github.com/repos/servo/servo/contents/{+path}",
+        "compare_url": "https://api.github.com/repos/servo/servo/compare/{base}...{head}",
+        "merges_url": "https://api.github.com/repos/servo/servo/merges",
+        "archive_url": "https://api.github.com/repos/servo/servo/{archive_format}{/ref}",
+        "downloads_url": "https://api.github.com/repos/servo/servo/downloads",
+        "issues_url": "https://api.github.com/repos/servo/servo/issues{/number}",
+        "pulls_url": "https://api.github.com/repos/servo/servo/pulls{/number}",
+        "milestones_url": "https://api.github.com/repos/servo/servo/milestones{/number}",
+        "notifications_url": "https://api.github.com/repos/servo/servo/notifications{?since,all,participating}",
+        "labels_url": "https://api.github.com/repos/servo/servo/labels{/name}",
+        "releases_url": "https://api.github.com/repos/servo/servo/releases{/id}",
+        "deployments_url": "https://api.github.com/repos/servo/servo/deployments",
+        "created_at": "2012-02-08T19:07:25Z",
+        "updated_at": "2017-12-21T09:43:03Z",
+        "pushed_at": "2017-12-21T13:12:14Z",
+        "git_url": "git://github.com/servo/servo.git",
+        "ssh_url": "git@github.com:servo/servo.git",
+        "clone_url": "https://github.com/servo/servo.git",
+        "svn_url": "https://github.com/servo/servo",
+        "homepage": "https://servo.org/",
+        "size": 402429,
+        "stargazers_count": 10867,
+        "watchers_count": 10867,
+        "language": "Rust",
+        "has_issues": true,
+        "has_projects": false,
+        "has_downloads": true,
+        "has_wiki": true,
+        "has_pages": false,
+        "forks_count": 1810,
+        "mirror_url": null,
+        "archived": false,
+        "open_issues_count": 2027,
+        "license": {
+          "key": "mpl-2.0",
+          "name": "Mozilla Public License 2.0",
+          "spdx_id": "MPL-2.0",
+          "url": "https://api.github.com/licenses/mpl-2.0"
+        },
+        "forks": 1810,
+        "open_issues": 2027,
+        "watchers": 10867,
+        "default_branch": "master"
+      }
+    },
+    "_links": {
+      "self": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/19612"
+      },
+      "html": {
+        "href": "https://github.com/servo/servo/pull/19612"
+      },
+      "issue": {
+        "href": "https://api.github.com/repos/servo/servo/issues/19612"
+      },
+      "comments": {
+        "href": "https://api.github.com/repos/servo/servo/issues/19612/comments"
+      },
+      "review_comments": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/19612/comments"
+      },
+      "review_comment": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/comments{/number}"
+      },
+      "commits": {
+        "href": "https://api.github.com/repos/servo/servo/pulls/19612/commits"
+      },
+      "statuses": {
+        "href": "https://api.github.com/repos/servo/servo/statuses/ab19bd1b6438c8e3aa0e5ab36dc3a6d5b14c12dd"
+      }
+    },
+    "author_association": "CONTRIBUTOR",
+    "merged": false,
+    "mergeable": null,
+    "rebaseable": null,
+    "mergeable_state": "unknown",
+    "merged_by": null,
+    "comments": 61,
+    "review_comments": 0,
+    "maintainer_can_modify": true,
+    "commits": 1,
+    "additions": 19,
+    "deletions": 20,
+    "changed_files": 8
+  },
+  "before": "537b653d9044d2f265d5790ba3761cea539a57f6",
+  "after": "ab19bd1b6438c8e3aa0e5ab36dc3a6d5b14c12dd",
+  "repository": {
+    "id": 3390243,
+    "name": "servo",
+    "full_name": "servo/servo",
+    "owner": {
+      "login": "servo",
+      "id": 2566135,
+      "avatar_url": "https://avatars1.githubusercontent.com/u/2566135?v=4",
+      "gravatar_id": "",
+      "url": "https://api.github.com/users/servo",
+      "html_url": "https://github.com/servo",
+      "followers_url": "https://api.github.com/users/servo/followers",
+      "following_url": "https://api.github.com/users/servo/following{/other_user}",
+      "gists_url": "https://api.github.com/users/servo/gists{/gist_id}",
+      "starred_url": "https://api.github.com/users/servo/starred{/owner}{/repo}",
+      "subscriptions_url": "https://api.github.com/users/servo/subscriptions",
+      "organizations_url": "https://api.github.com/users/servo/orgs",
+      "repos_url": "https://api.github.com/users/servo/repos",
+      "events_url": "https://api.github.com/users/servo/events{/privacy}",
+      "received_events_url": "https://api.github.com/users/servo/received_events",
+      "type": "Organization",
+      "site_admin": false
+    },
+    "private": false,
+    "html_url": "https://github.com/servo/servo",
+    "description": "The Servo Browser Engine",
+    "fork": false,
+    "url": "https://api.github.com/repos/servo/servo",
+    "forks_url": "https://api.github.com/repos/servo/servo/forks",
+    "keys_url": "https://api.github.com/repos/servo/servo/keys{/key_id}",
+    "collaborators_url": "https://api.github.com/repos/servo/servo/collaborators{/collaborator}",
+    "teams_url": "https://api.github.com/repos/servo/servo/teams",
+    "hooks_url": "https://api.github.com/repos/servo/servo/hooks",
+    "issue_events_url": "https://api.github.com/repos/servo/servo/issues/events{/number}",
+    "events_url": "https://api.github.com/repos/servo/servo/events",
+    "assignees_url": "https://api.github.com/repos/servo/servo/assignees{/user}",
+    "branches_url": "https://api.github.com/repos/servo/servo/branches{/branch}",
+    "tags_url": "https://api.github.com/repos/servo/servo/tags",
+    "blobs_url": "https://api.github.com/repos/servo/servo/git/blobs{/sha}",
+    "git_tags_url": "https://api.github.com/repos/servo/servo/git/tags{/sha}",
+    "git_refs_url": "https://api.github.com/repos/servo/servo/git/refs{/sha}",
+    "trees_url": "https://api.github.com/repos/servo/servo/git/trees{/sha}",
+    "statuses_url": "https://api.github.com/repos/servo/servo/statuses/{sha}",
+    "languages_url": "https://api.github.com/repos/servo/servo/languages",
+    "stargazers_url": "https://api.github.com/repos/servo/servo/stargazers",
+    "contributors_url": "https://api.github.com/repos/servo/servo/contributors",
+    "subscribers_url": "https://api.github.com/repos/servo/servo/subscribers",
+    "subscription_url": "https://api.github.com/repos/servo/servo/subscription",
+    "commits_url": "https://api.github.com/repos/servo/servo/commits{/sha}",
+    "git_commits_url": "https://api.github.com/repos/servo/servo/git/commits{/sha}",
+    "comments_url": "https://api.github.com/repos/servo/servo/comments{/number}",
+    "issue_comment_url": "https://api.github.com/repos/servo/servo/issues/comments{/number}",
+    "contents_url": "https://api.github.com/repos/servo/servo/contents/{+path}",
+    "compare_url": "https://api.github.com/repos/servo/servo/compare/{base}...{head}",
+    "merges_url": "https://api.github.com/repos/servo/servo/merges",
+    "archive_url": "https://api.github.com/repos/servo/servo/{archive_format}{/ref}",
+    "downloads_url": "https://api.github.com/repos/servo/servo/downloads",
+    "issues_url": "https://api.github.com/repos/servo/servo/issues{/number}",
+    "pulls_url": "https://api.github.com/repos/servo/servo/pulls{/number}",
+    "milestones_url": "https://api.github.com/repos/servo/servo/milestones{/number}",
+    "notifications_url": "https://api.github.com/repos/servo/servo/notifications{?since,all,participating}",
+    "labels_url": "https://api.github.com/repos/servo/servo/labels{/name}",
+    "releases_url": "https://api.github.com/repos/servo/servo/releases{/id}",
+    "deployments_url": "https://api.github.com/repos/servo/servo/deployments",
+    "created_at": "2012-02-08T19:07:25Z",
+    "updated_at": "2017-12-21T09:43:03Z",
+    "pushed_at": "2017-12-21T13:12:14Z",
+    "git_url": "git://github.com/servo/servo.git",
+    "ssh_url": "git@github.com:servo/servo.git",
+    "clone_url": "https://github.com/servo/servo.git",
+    "svn_url": "https://github.com/servo/servo",
+    "homepage": "https://servo.org/",
+    "size": 402429,
+    "stargazers_count": 10867,
+    "watchers_count": 10867,
+    "language": "Rust",
+    "has_issues": true,
+    "has_projects": false,
+    "has_downloads": true,
+    "has_wiki": true,
+    "has_pages": false,
+    "forks_count": 1810,
+    "mirror_url": null,
+    "archived": false,
+    "open_issues_count": 2027,
+    "license": {
+      "key": "mpl-2.0",
+      "name": "Mozilla Public License 2.0",
+      "spdx_id": "MPL-2.0",
+      "url": "https://api.github.com/licenses/mpl-2.0"
+    },
+    "forks": 1810,
+    "open_issues": 2027,
+    "watchers": 10867,
+    "default_branch": "master"
+  },
+  "organization": {
+    "login": "servo",
+    "id": 2566135,
+    "url": "https://api.github.com/orgs/servo",
+    "repos_url": "https://api.github.com/orgs/servo/repos",
+    "events_url": "https://api.github.com/orgs/servo/events",
+    "hooks_url": "https://api.github.com/orgs/servo/hooks",
+    "issues_url": "https://api.github.com/orgs/servo/issues",
+    "members_url": "https://api.github.com/orgs/servo/members{/member}",
+    "public_members_url": "https://api.github.com/orgs/servo/public_members{/member}",
+    "avatar_url": "https://avatars1.githubusercontent.com/u/2566135?v=4",
+    "description": "The Servo web browser engine"
+  },
+  "sender": {
+    "login": "tigercosmos",
+    "id": 18013815,
+    "avatar_url": "https://avatars0.githubusercontent.com/u/18013815?v=4",
+    "gravatar_id": "",
+    "url": "https://api.github.com/users/tigercosmos",
+    "html_url": "https://github.com/tigercosmos",
+    "followers_url": "https://api.github.com/users/tigercosmos/followers",
+    "following_url": "https://api.github.com/users/tigercosmos/following{/other_user}",
+    "gists_url": "https://api.github.com/users/tigercosmos/gists{/gist_id}",
+    "starred_url": "https://api.github.com/users/tigercosmos/starred{/owner}{/repo}",
+    "subscriptions_url": "https://api.github.com/users/tigercosmos/subscriptions",
+    "organizations_url": "https://api.github.com/users/tigercosmos/orgs",
+    "repos_url": "https://api.github.com/users/tigercosmos/repos",
+    "events_url": "https://api.github.com/users/tigercosmos/events{/privacy}",
+    "received_events_url": "https://api.github.com/users/tigercosmos/received_events",
+    "type": "User",
+    "site_admin": false
+  }
+}
diff --git a/etc/ci/upstream-wpt-changes/tests/wpt-mock/css/css-test.html b/etc/ci/upstream-wpt-changes/tests/wpt-mock/css/css-test.html
new file mode 100644
index 00000000000..cffb6eb6b5a
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/wpt-mock/css/css-test.html
@@ -0,0 +1,3 @@
+<html>
+<h1>css test!</h1>
+</html>
diff --git a/etc/ci/upstream-wpt-changes/tests/wpt-mock/fetch/api/redirect/redirect-location.html b/etc/ci/upstream-wpt-changes/tests/wpt-mock/fetch/api/redirect/redirect-location.html
new file mode 100644
index 00000000000..48a68cd53ce
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/wpt-mock/fetch/api/redirect/redirect-location.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Fetch: handling Location header during redirection</title>
+    <meta name="author" title="Canon Research France" href="https://www.crf.canon.fr">
+    <meta name="help" href="https://fetch.spec.whatwg.org/#http-network-or-cache-fetch">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+  </head>
+  <body>
+    <script src="../resources/utils.js"></script>
+    <script src="redirect-location.js"></script>
+  </body>
+</html>
+
diff --git a/etc/ci/upstream-wpt-changes/tests/wpt-mock/test.html b/etc/ci/upstream-wpt-changes/tests/wpt-mock/test.html
new file mode 100644
index 00000000000..43b09d4e57d
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/wpt-mock/test.html
@@ -0,0 +1,3 @@
+<html>
+<h1>this is a test file</h1>
+</html>
diff --git a/etc/ci/upstream-wpt-changes/tests/wpt.diff b/etc/ci/upstream-wpt-changes/tests/wpt.diff
new file mode 100644
index 00000000000..37781cf488e
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/tests/wpt.diff
@@ -0,0 +1,9 @@
+diff --git a/tests/wpt/web-platform-tests/css/css-test.html b/tests/wpt/web-platform-tests/css/css-test.html
+index cffb6eb..6100a49 100644
+--- a/tests/wpt/web-platform-tests/css/css-test.html
++++ b/tests/wpt/web-platform-tests/css/css-test.html
+@@ -1,3 +1,3 @@
+ <html>
+-<h1>css test!</h1>
++<h1>css test!</h1>hi
+ </html>
diff --git a/etc/ci/upstream-wpt-changes/upstream-wpt-changes.py b/etc/ci/upstream-wpt-changes/upstream-wpt-changes.py
new file mode 100755
index 00000000000..9c5c11ec3b1
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/upstream-wpt-changes.py
@@ -0,0 +1,43 @@
+#!/usr/bin/env python
+
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+# pylint: disable=missing-docstring
+# pylint: disable=invalid-name
+
+import json
+import logging
+import os
+import sys
+
+from wptupstreamer import WPTSync
+
+
+def main() -> int:
+    context = json.loads(os.environ['GITHUB_CONTEXT'])
+    logging.getLogger().level = logging.INFO
+
+    success = WPTSync(
+        servo_repo='servo/servo',
+        wpt_repo='servo/wpt',
+        downstream_wpt_repo='servo-wpt-sync/web-platform-tests',
+        servo_path='./servo',
+        wpt_path='./wpt',
+        github_api_token=os.environ['WPT_SYNC_GITHUB_TOKEN'],
+        github_api_url='https://api.github.com/',
+        github_username='servo-wpt-sync',
+        github_email='josh+wptsync@joshmatthews.net',
+        github_name='Servo WPT Sync',
+    ).run(context["event"])
+    return 0 if success else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py b/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py
new file mode 100644
index 00000000000..97b0de8af72
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/wptupstreamer/__init__.py
@@ -0,0 +1,268 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+# pylint: disable=broad-except
+# pylint: disable=dangerous-default-value
+# pylint: disable=fixme
+# pylint: disable=missing-docstring
+
+# This allows using types that are defined later in the file.
+from __future__ import annotations
+
+import dataclasses
+import json
+import logging
+import re
+import subprocess
+
+from typing import Callable, Optional
+
+from .common import \
+    CLOSING_EXISTING_UPSTREAM_PR, \
+    NO_SYNC_SIGNAL, \
+    NO_UPSTREAMBLE_CHANGES_COMMENT, \
+    OPENED_NEW_UPSTREAM_PR, \
+    UPDATED_EXISTING_UPSTREAM_PR, \
+    UPDATED_TITLE_IN_EXISTING_UPSTREAM_PR, \
+    UPSTREAMABLE_PATH, \
+    wpt_branch_name_from_servo_pr_number
+
+from .github import GithubRepository, PullRequest
+from .step import \
+    AsyncValue, \
+    ChangePRStep, \
+    CommentStep, \
+    CreateOrUpdateBranchForPRStep, \
+    MergePRStep, \
+    OpenPRStep, \
+    RemoveBranchForPRStep, \
+    Step
+
+
+class LocalGitRepo:
+    def __init__(self, path: str, sync: WPTSync):
+        self.path = path
+        self.sync = sync
+
+    def run(self, *args, env: dict = {}):
+        command_line = ["git"] + list(args)
+        logging.info("  → Execution (cwd='%s'): %s",
+                     self.path, " ".join(command_line))
+
+        env.setdefault("GIT_AUTHOR_EMAIL", self.sync.github_email)
+        env.setdefault("GIT_COMMITTER_EMAIL", self.sync.github_email)
+        env.setdefault("GIT_AUTHOR_NAME", self.sync.github_name)
+        env.setdefault("GIT_COMMITTER_NAME", self.sync.github_name)
+
+        return subprocess.check_output(
+            command_line, cwd=self.path, env=env, stderr=subprocess.STDOUT
+        ).decode("utf-8")
+
+
+@dataclasses.dataclass()
+class SyncRun:
+    sync: WPTSync
+    servo_pr: PullRequest
+    upstream_pr: AsyncValue[PullRequest]
+    step_callback: Optional[Callable[[Step], None]]
+    steps: list[Step] = dataclasses.field(default_factory=list)
+
+    def make_comment(self, template: str) -> str:
+        upstream_pr = self.upstream_pr.value() if self.upstream_pr.has_value() else ""
+        return template.format(
+            upstream_pr=upstream_pr,
+            servo_pr=self.servo_pr,
+        )
+
+    def add_step(self, step) -> Optional[AsyncValue]:
+        self.steps.append(step)
+        return step.provides()
+
+    def run(self):
+        # This loop always removes the first step and runs it, because
+        # individual steps can modify the list of steps. For instance, if a
+        # step fails, it might clear the remaining steps and replace them with
+        # steps that report the error to GitHub.
+        while self.steps:
+            step = self.steps.pop(0)
+            step.run(self)
+            if self.step_callback:
+                self.step_callback(step)
+
+    @staticmethod
+    def clean_up_body_text(body: str) -> str:
+        # Turn all bare or relative issue references into unlinked ones, so that
+        # the PR doesn't inadvertently close or link to issues in the upstream
+        # repository.
+        return (
+            re.sub(
+                r"(^|\s)(\w*)#([1-9]\d*)",
+                r"\g<1>\g<2>#<!-- nolink -->\g<3>",
+                body,
+                flags=re.MULTILINE,
+            )
+            .split("\n---")[0]
+            .split("<!-- Thank you for")[0]
+        )
+
+    def prepare_body_text(self, body: str) -> str:
+        return SyncRun.clean_up_body_text(body) + f"\nReviewed in {self.servo_pr}"
+
+
+@dataclasses.dataclass(kw_only=True)
+class WPTSync:
+    servo_repo: str
+    wpt_repo: str
+    downstream_wpt_repo: str
+    servo_path: str
+    wpt_path: str
+    github_api_token: str
+    github_api_url: str
+    github_username: str
+    github_email: str
+    github_name: str
+    suppress_force_push: bool = False
+
+    def __post_init__(self):
+        self.servo = GithubRepository(self, self.servo_repo)
+        self.wpt = GithubRepository(self, self.wpt_repo)
+        self.downstream_wpt = GithubRepository(self, self.downstream_wpt_repo)
+        self.local_servo_repo = LocalGitRepo(self.servo_path, self)
+        self.local_wpt_repo = LocalGitRepo(self.wpt_path, self)
+
+    def run(self, payload: dict, step_callback=None) -> bool:
+        if "pull_request" not in payload:
+            return True
+
+        pull_data = payload["pull_request"]
+        if NO_SYNC_SIGNAL in pull_data.get("body", ""):
+            return True
+
+        # Only look for an existing remote PR if the action is appropriate.
+        logging.info("Processing '%s' action...", payload["action"])
+        action = payload["action"]
+        if action not in ["opened", "synchronize", "reopened", "edited", "closed"]:
+            return True
+
+        if (
+            action == "edited"
+            and "title" not in payload["changes"]
+            and "body" not in payload["changes"]
+        ):
+            return True
+
+        try:
+            servo_pr = self.servo.get_pull_request(pull_data["number"])
+            downstream_wpt_branch = self.downstream_wpt.get_branch(
+                wpt_branch_name_from_servo_pr_number(servo_pr.number)
+            )
+            upstream_pr = self.wpt.get_open_pull_request_for_branch(
+                downstream_wpt_branch
+            )
+            if upstream_pr:
+                logging.info(
+                    "  → Detected existing upstream PR %s", upstream_pr)
+
+            run = SyncRun(self, servo_pr, AsyncValue(upstream_pr), step_callback)
+
+            pull_data = payload["pull_request"]
+            if payload["action"] in ["opened", "synchronize", "reopened"]:
+                self.handle_new_pull_request_contents(run, pull_data)
+            elif payload["action"] == "edited":
+                self.handle_edited_pull_request(run, pull_data)
+            elif payload["action"] == "closed":
+                self.handle_closed_pull_request(run, pull_data)
+
+            run.run()
+            return True
+        except Exception as exception:
+            if isinstance(exception, subprocess.CalledProcessError):
+                logging.error(exception.output)
+            logging.error(json.dumps(payload))
+            logging.error(exception, exc_info=True)
+            return False
+
+    def handle_new_pull_request_contents(self, run: SyncRun, pull_data: dict):
+        is_upstreamable = (
+            len(
+                self.local_servo_repo.run(
+                    "diff", f"HEAD~{pull_data['commits']}", "--", UPSTREAMABLE_PATH
+                )
+            )
+            > 0
+        )
+        logging.info("  → PR is upstreamable: '%s'", is_upstreamable)
+
+        title = pull_data['title']
+        body = pull_data['body']
+        if run.upstream_pr.has_value():
+            if is_upstreamable:
+                # In case this is adding new upstreamable changes to a PR that was closed
+                # due to a lack of upstreamable changes, force it to be reopened.
+                # Github refuses to reopen a PR that had a branch force pushed, so be sure
+                # to do this first.
+                run.add_step(ChangePRStep(
+                    run.upstream_pr.value(), "opened", title, body))
+                # Push the relevant changes to the upstream branch.
+                run.add_step(CreateOrUpdateBranchForPRStep(
+                    pull_data, run.servo_pr))
+                run.add_step(CommentStep(
+                    run.servo_pr, UPDATED_EXISTING_UPSTREAM_PR))
+            else:
+                # Close the upstream PR, since would contain no changes otherwise.
+                run.add_step(CommentStep(run.upstream_pr.value(),
+                             NO_UPSTREAMBLE_CHANGES_COMMENT))
+                run.add_step(ChangePRStep(run.upstream_pr.value(), "closed"))
+                run.add_step(RemoveBranchForPRStep(pull_data))
+                run.add_step(CommentStep(
+                    run.servo_pr, CLOSING_EXISTING_UPSTREAM_PR))
+
+        elif is_upstreamable:
+            # Push the relevant changes to a new upstream branch.
+            branch = run.add_step(
+                CreateOrUpdateBranchForPRStep(pull_data, run.servo_pr))
+
+            # Create a pull request against the upstream repository for the new branch.
+            assert branch
+            upstream_pr = run.add_step(OpenPRStep(
+                branch, self.wpt, title, body,
+                ["servo-export", "do not merge yet"],
+            ))
+
+            assert upstream_pr
+            run.upstream_pr = upstream_pr
+
+            # Leave a comment to the new pull request in the original pull request.
+            run.add_step(CommentStep(run.servo_pr, OPENED_NEW_UPSTREAM_PR))
+
+    def handle_edited_pull_request(self, run: SyncRun, pull_data: dict):
+        logging.info("Changing upstream PR title")
+        if run.upstream_pr.has_value():
+            run.add_step(ChangePRStep(
+                run.upstream_pr.value(), "open", pull_data["title"], pull_data["body"]
+            ))
+            run.add_step(CommentStep(
+                run.servo_pr, UPDATED_TITLE_IN_EXISTING_UPSTREAM_PR))
+
+    def handle_closed_pull_request(self, run: SyncRun, pull_data: dict):
+        logging.info("Processing closed PR")
+        if not run.upstream_pr.has_value():
+            # If we don't recognize this PR, it never contained upstreamable changes.
+            return
+        if pull_data["merged"]:
+            # Since the upstreamable changes have now been merged locally, merge the
+            # corresponding upstream PR.
+            run.add_step(MergePRStep(run.upstream_pr.value(), ["do not merge yet"]))
+        else:
+            # If a PR with upstreamable changes is closed without being merged, we
+            # don't want to merge the changes upstream either.
+            run.add_step(ChangePRStep(run.upstream_pr.value(), "closed"))
+
+        # Always clean up our remote branch.
+        run.add_step(RemoveBranchForPRStep(pull_data))
diff --git a/etc/ci/upstream-wpt-changes/wptupstreamer/common.py b/etc/ci/upstream-wpt-changes/wptupstreamer/common.py
new file mode 100644
index 00000000000..307edce0276
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/wptupstreamer/common.py
@@ -0,0 +1,53 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+# pylint: disable=missing-docstring
+
+UPSTREAMABLE_PATH = "tests/wpt/web-platform-tests/"
+NO_SYNC_SIGNAL = "[no-wpt-sync]"
+
+OPENED_NEW_UPSTREAM_PR = (
+    "🤖 Opened new upstream WPT pull request ({upstream_pr}) "
+    "with upstreamable changes."
+)
+UPDATED_EXISTING_UPSTREAM_PR = (
+    "📝 Transplanted new upstreamable changes to existing "
+    "upstream WPT pull request ({upstream_pr})."
+)
+UPDATED_TITLE_IN_EXISTING_UPSTREAM_PR = (
+    "✍ Updated existing upstream WPT pull request ({upstream_pr}) title and body."
+)
+CLOSING_EXISTING_UPSTREAM_PR = (
+    "🤖 This change no longer contains upstreamable changes to WPT; closed existing "
+    "upstream pull request ({upstream_pr})."
+)
+NO_UPSTREAMBLE_CHANGES_COMMENT = (
+    "👋 Downstream pull request ({servo_pr}) no longer contains any upstreamable "
+    "changes. Closing pull request without merging."
+)
+COULD_NOT_APPLY_CHANGES_DOWNSTREAM_COMMENT = (
+    "🛠 These changes could not be applied onto the latest upstream WPT. "
+    "Servo's copy of the Web Platform Tests may be out of sync."
+)
+COULD_NOT_APPLY_CHANGES_UPSTREAM_COMMENT = (
+    "🛠 Changes from the source pull request ({servo_pr}) can no longer be "
+    "cleanly applied. Waiting for a new version of these changes downstream."
+)
+COULD_NOT_MERGE_CHANGES_DOWNSTREAM_COMMENT = (
+    "⛔ Failed to properly merge the upstream pull request ({upstream_pr}). "
+    "Please address any CI issues and try to merge manually."
+)
+COULD_NOT_MERGE_CHANGES_UPSTREAM_COMMENT = (
+    "⛔ The downstream PR has merged ({servo_pr}), but these changes could not "
+    "be merged properly. Please address any CI issues and try to merge manually."
+)
+
+
+def wpt_branch_name_from_servo_pr_number(servo_pr_number):
+    return f"servo_export_{servo_pr_number}"
diff --git a/etc/ci/upstream-wpt-changes/wptupstreamer/github.py b/etc/ci/upstream-wpt-changes/wptupstreamer/github.py
new file mode 100644
index 00000000000..4905a841fe2
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/wptupstreamer/github.py
@@ -0,0 +1,166 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+# pylint: disable=missing-docstring
+
+"""This modules contains some abstractions of GitHub repositories. It could one
+day be entirely replaced with something like PyGithub."""
+
+# This allows using types that are defined later in the file.
+from __future__ import annotations
+
+import logging
+import urllib
+
+from typing import Optional, TYPE_CHECKING
+
+import requests
+
+if TYPE_CHECKING:
+    from . import WPTSync
+
+USER_AGENT = "Servo web-platform-test sync service"
+TIMEOUT = 30  # 30 seconds
+
+
+def authenticated(sync: WPTSync, method, url, json=None) -> requests.Response:
+    logging.info("  → Request: %s %s", method, url)
+    if json:
+        logging.info("  → Request JSON: %s", json)
+
+    headers = {
+        "Authorization": f"Bearer {sync.github_api_token}",
+        "User-Agent": USER_AGENT,
+    }
+
+    url = urllib.parse.urljoin(sync.github_api_url, url)
+    response = requests.request(
+        method, url, headers=headers, json=json, timeout=TIMEOUT
+    )
+    if int(response.status_code / 100) != 2:
+        raise ValueError(
+            f"Got unexpected {response.status_code} response: {response.text}"
+        )
+    return response
+
+
+class GithubRepository:
+    """
+    This class allows interacting with a single GitHub repository.
+    """
+
+    def __init__(self, sync: WPTSync, repo: str):
+        self.sync = sync
+        self.repo = repo
+        self.org = repo.split("/")[0]
+        self.pulls_url = f"repos/{self.repo}/pulls"
+
+    def __str__(self):
+        return self.repo
+
+    def get_pull_request(self, number: int) -> PullRequest:
+        return PullRequest(self, number)
+
+    def get_branch(self, name: str) -> GithubBranch:
+        return GithubBranch(self, name)
+
+    def get_open_pull_request_for_branch(
+        self, branch: GithubBranch
+    ) -> Optional[PullRequest]:
+        """If this repository has an open pull request with the
+        given source head reference targeting the master branch,
+        return the first matching pull request, otherwise return None."""
+        # Frustratingly, this is different from what you need for opening a pull request.
+        head = f"{branch.repo.org}:{branch.name}"
+        response = authenticated(
+            self.sync, "GET", f"{self.pulls_url}?head={head}&base=master&state=open"
+        )
+        if int(response.status_code / 100) != 2:
+            return None
+
+        json = response.json()
+        if not json or not isinstance(json, list):
+            return None
+        return self.get_pull_request(json[0]["number"])
+
+    def open_pull_request(self, branch: GithubBranch, title: str, body: str):
+        data = {
+            "title": title,
+            "head": branch.get_pr_head_reference_for_repo(self),
+            "base": "master",
+            "body": body,
+            "maintainer_can_modify": False,
+        }
+        response = authenticated(self.sync, "POST", self.pulls_url, json=data)
+        return self.get_pull_request(response.json()["number"])
+
+
+class GithubBranch:
+    def __init__(self, repo: GithubRepository, branch_name: str):
+        self.repo = repo
+        self.name = branch_name
+
+    def __str__(self):
+        return f"{self.repo}/{self.name}"
+
+    def get_pr_head_reference_for_repo(self, other_repo: GithubRepository) -> str:
+        """Get the head reference to use in pull requests for the given repository.
+        If the organization is the same this is just `<branch>` otherwise
+        it will be `<org>:<branch>`."""
+        if self.repo.org == other_repo.org:
+            return self.name
+        return f"{self.repo.org}:{self.name}"
+
+
+class PullRequest:
+    """
+    This class allows interacting with a single pull request on GitHub.
+    """
+
+    def __init__(self, repo: GithubRepository, number: int):
+        self.repo = repo
+        self.context = repo.sync
+        self.number = number
+        self.base_url = f"repos/{self.repo.repo}/pulls/{self.number}"
+        self.base_issues_url = f"repos/{self.repo.repo}/issues/{self.number}"
+
+    def __str__(self):
+        return f"{self.repo}#{self.number}"
+
+    def api(self, *args, **kwargs) -> requests.Response:
+        return authenticated(self.context, *args, **kwargs)
+
+    def leave_comment(self, comment: str):
+        return self.api(
+            "POST", f"{self.base_issues_url}/comments", json={"body": comment}
+        )
+
+    def change(
+        self,
+        state: Optional[str] = None,
+        title: Optional[str] = None,
+        body: Optional[str] = None,
+    ):
+        data = {}
+        if title:
+            data["title"] = title
+        if body:
+            data["body"] = body
+        if state:
+            data["state"] = state
+        return self.api("PATCH", self.base_url, json=data)
+
+    def remove_label(self, label: str):
+        self.api("DELETE", f"{self.base_issues_url}/labels/{label}")
+
+    def add_labels(self, labels: list[str]):
+        self.api("POST", f"{self.base_issues_url}/labels", json=labels)
+
+    def merge(self):
+        self.api("PUT", f"{self.base_url}/merge", json={"merge_method": "rebase"})
diff --git a/etc/ci/upstream-wpt-changes/wptupstreamer/step.py b/etc/ci/upstream-wpt-changes/wptupstreamer/step.py
new file mode 100644
index 00000000000..31ae94b96eb
--- /dev/null
+++ b/etc/ci/upstream-wpt-changes/wptupstreamer/step.py
@@ -0,0 +1,300 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+# pylint: disable=broad-except
+# pylint: disable=dangerous-default-value
+# pylint: disable=fixme
+# pylint: disable=missing-docstring
+
+# This allows using types that are defined later in the file.
+from __future__ import annotations
+
+import logging
+import os
+import textwrap
+
+from typing import TYPE_CHECKING, Generic, Optional, TypeVar
+
+from .common import COULD_NOT_APPLY_CHANGES_DOWNSTREAM_COMMENT
+from .common import COULD_NOT_APPLY_CHANGES_UPSTREAM_COMMENT
+from .common import COULD_NOT_MERGE_CHANGES_DOWNSTREAM_COMMENT
+from .common import COULD_NOT_MERGE_CHANGES_UPSTREAM_COMMENT
+from .common import UPSTREAMABLE_PATH
+from .common import wpt_branch_name_from_servo_pr_number
+from .github import GithubBranch, GithubRepository, PullRequest
+
+if TYPE_CHECKING:
+    from . import SyncRun, WPTSync
+
+PATCH_FILE_NAME = "tmp.patch"
+
+
+class Step:
+    def __init__(self, name):
+        self.name = name
+
+    def provides(self) -> Optional[AsyncValue]:
+        return None
+
+    def run(self, _: SyncRun):
+        return
+
+
+T = TypeVar('T')
+
+
+class AsyncValue(Generic[T]):
+    def __init__(self, value: Optional[T] = None):
+        self._value = value
+
+    def resolve(self, value: T):
+        self._value = value
+
+    def value(self) -> T:
+        assert self._value is not None
+        return self._value
+
+    def has_value(self):
+        return self._value is not None
+
+
+class CreateOrUpdateBranchForPRStep(Step):
+    def __init__(self, pull_data: dict, pull_request: PullRequest):
+        Step.__init__(self, "CreateOrUpdateBranchForPRStep")
+        self.pull_data = pull_data
+        self.pull_request = pull_request
+        self.branch: AsyncValue[GithubBranch] = AsyncValue()
+
+    def provides(self):
+        return self.branch
+
+    def run(self, run: SyncRun):
+        try:
+            commits = self._get_upstreamable_commits_from_local_servo_repo(
+                run.sync)
+            branch_name = self._create_or_update_branch_for_pr(run, commits)
+            branch = run.sync.downstream_wpt.get_branch(branch_name)
+
+            self.branch.resolve(branch)
+            self.name += f":{len(commits)}:{branch}"
+        except Exception as exception:
+            logging.info("Could not apply changes to upstream WPT repository.")
+            logging.info(exception, exc_info=True)
+
+            run.steps = []
+            run.add_step(CommentStep(
+                self.pull_request, COULD_NOT_APPLY_CHANGES_DOWNSTREAM_COMMENT
+            ))
+            if run.upstream_pr.has_value():
+                run.add_step(CommentStep(
+                    run.upstream_pr.value(), COULD_NOT_APPLY_CHANGES_UPSTREAM_COMMENT
+                ))
+
+    def _get_upstreamable_commits_from_local_servo_repo(self, sync: WPTSync):
+        local_servo_repo = sync.local_servo_repo
+        number_of_commits = self.pull_data["commits"]
+        commit_shas = local_servo_repo.run(
+            "log", "--pretty=%H", f"-{number_of_commits}"
+        ).splitlines()
+
+        filtered_commits = []
+        for sha in commit_shas:
+            # Specifying the path here does a few things. First, it excludes any
+            # changes that do not touch WPT files at all. Secondly, when a file is
+            # moved in or out of the WPT directory the filename which is outside the
+            # directory becomes /dev/null and the change becomes an addition or
+            # deletion. This makes the patch usable on the WPT repository itself.
+            # TODO: If we could cleverly parse and manipulate the full commit diff
+            # we could avoid cloning the servo repository altogether and only
+            # have to fetch the commit diffs from GitHub.
+            diff = local_servo_repo.run(
+                "show", "--binary", "--format=%b", sha, "--", UPSTREAMABLE_PATH
+            )
+
+            # Retrieve the diff of any changes to files that are relevant
+            if diff:
+                # Create an object that contains everything necessary to transplant this
+                # commit to another repository.
+                filtered_commits += [
+                    {
+                        "author": local_servo_repo.run(
+                            "show", "-s", "--pretty=%an <%ae>", sha
+                        ),
+                        "message": local_servo_repo.run(
+                            "show", "-s", "--pretty=%B", sha
+                        ),
+                        "diff": diff,
+                    }
+                ]
+        return filtered_commits
+
+    def _apply_filtered_servo_commit_to_wpt(self, run: SyncRun, commit: dict):
+        patch_path = os.path.join(run.sync.wpt_path, PATCH_FILE_NAME)
+        strip_count = UPSTREAMABLE_PATH.count("/") + 1
+
+        try:
+            with open(patch_path, "w", encoding="utf-8") as file:
+                file.write(commit["diff"])
+            run.sync.local_wpt_repo.run(
+                "apply", PATCH_FILE_NAME, "-p", str(strip_count)
+            )
+        finally:
+            # Ensure the patch file is not added with the other changes.
+            os.remove(patch_path)
+
+        run.sync.local_wpt_repo.run("add", "--all")
+        run.sync.local_wpt_repo.run(
+            "commit", "--message", commit["message"], "--author", commit["author"]
+        )
+
+    def _create_or_update_branch_for_pr(
+        self, run: SyncRun, commits: list[dict], pre_commit_callback=None
+    ):
+        branch_name = wpt_branch_name_from_servo_pr_number(
+            self.pull_data["number"])
+        try:
+            # Create a new branch with a unique name that is consistent between
+            # updates of the same PR.
+            run.sync.local_wpt_repo.run("checkout", "-b", branch_name)
+
+            for commit in commits:
+                self._apply_filtered_servo_commit_to_wpt(run, commit)
+
+            if pre_commit_callback:
+                pre_commit_callback()
+
+            # Push the branch upstream (forcing to overwrite any existing changes).
+            if not run.sync.suppress_force_push:
+                user = run.sync.github_username
+                token = run.sync.github_api_token
+                repo = run.sync.downstream_wpt_repo
+                remote_url = f"https://{user}:{token}@github.com/{repo}.git"
+                run.sync.local_wpt_repo.run(
+                    "push", "-f", remote_url, branch_name)
+
+            return branch_name
+        finally:
+            try:
+                run.sync.local_wpt_repo.run("checkout", "master")
+                run.sync.local_wpt_repo.run("branch", "-D", branch_name)
+            except Exception:
+                pass
+
+
+class RemoveBranchForPRStep(Step):
+    def __init__(self, pull_request):
+        Step.__init__(self, "RemoveBranchForPRStep")
+        self.branch_name = wpt_branch_name_from_servo_pr_number(
+            pull_request["number"])
+
+    def run(self, run: SyncRun):
+        self.name += f":{run.sync.downstream_wpt.get_branch(self.branch_name)}"
+        logging.info("  -> Removing branch used for upstream PR")
+        if not run.sync.suppress_force_push:
+            run.sync.local_wpt_repo.run("push", "origin", "--delete",
+                                        self.branch_name)
+
+
+class ChangePRStep(Step):
+    def __init__(
+        self,
+        pull_request: PullRequest,
+        state: str,
+        title: Optional[str] = None,
+        body: Optional[str] = None,
+    ):
+        name = f"ChangePRStep:{pull_request}:{state}"
+        if title:
+            name += f":{title}"
+
+        Step.__init__(self, name)
+        self.pull_request = pull_request
+        self.state = state
+        self.title = title
+        self.body = body
+
+    def run(self, run: SyncRun):
+        body = self.body
+        if body:
+            body = run.prepare_body_text(body)
+            self.name += (
+                f':{textwrap.shorten(body, width=20, placeholder="...")}[{len(body)}]'
+            )
+
+        self.pull_request.change(state=self.state, title=self.title, body=body)
+
+
+class MergePRStep(Step):
+    def __init__(self, pull_request: PullRequest, labels_to_remove: list[str] = []):
+        Step.__init__(self, f"MergePRStep:{pull_request}")
+        self.pull_request = pull_request
+        self.labels_to_remove = labels_to_remove
+
+    def run(self, run: SyncRun):
+        for label in self.labels_to_remove:
+            self.pull_request.remove_label(label)
+        try:
+            self.pull_request.merge()
+        except Exception as exception:
+            logging.warning("Could not merge PR (%s).", self.pull_request)
+            logging.warning(exception, exc_info=True)
+
+            run.steps = []
+            run.add_step(CommentStep(
+                self.pull_request, COULD_NOT_MERGE_CHANGES_UPSTREAM_COMMENT
+            ))
+            run.add_step(CommentStep(
+                run.servo_pr, COULD_NOT_MERGE_CHANGES_DOWNSTREAM_COMMENT
+            ))
+            self.pull_request.add_labels(["stale-servo-export"])
+
+
+class OpenPRStep(Step):
+    def __init__(
+        self,
+        source_branch: AsyncValue[GithubBranch],
+        target_repo: GithubRepository,
+        title: str,
+        body: str,
+        labels: list[str],
+    ):
+        Step.__init__(self, "OpenPRStep")
+        self.title = title
+        self.body = body
+        self.source_branch = source_branch
+        self.target_repo = target_repo
+        self.new_pr: AsyncValue[PullRequest] = AsyncValue()
+        self.labels = labels
+
+    def provides(self):
+        return self.new_pr
+
+    def run(self, run: SyncRun):
+        pull_request = self.target_repo.open_pull_request(
+            self.source_branch.value(), self.title, run.prepare_body_text(self.body)
+        )
+
+        if self.labels:
+            pull_request.add_labels(self.labels)
+
+        self.new_pr.resolve(pull_request)
+
+        self.name += f":{self.source_branch.value()}→{self.new_pr.value()}"
+
+
+class CommentStep(Step):
+    def __init__(self, pull_request: PullRequest, comment_template: str):
+        Step.__init__(self, "CommentStep")
+        self.pull_request = pull_request
+        self.comment_template = comment_template
+
+    def run(self, run: SyncRun):
+        comment = run.make_comment(self.comment_template)
+        self.name += f":{self.pull_request}:{comment}"
+        self.pull_request.leave_comment(comment)