diff --git a/tests/wpt/metadata-layout-2020/css/CSS2/floats/hit-test-floats-002.html.ini b/tests/wpt/metadata-layout-2020/css/CSS2/floats/hit-test-floats-002.html.ini
deleted file mode 100644
index f64b45fea6b..00000000000
--- a/tests/wpt/metadata-layout-2020/css/CSS2/floats/hit-test-floats-002.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[hit-test-floats-002.html]
-  [Hit test float]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata-layout-2020/css/CSS2/floats/hit-test-floats-005.html.ini b/tests/wpt/metadata-layout-2020/css/CSS2/floats/hit-test-floats-005.html.ini
deleted file mode 100644
index baa9f1a7541..00000000000
--- a/tests/wpt/metadata-layout-2020/css/CSS2/floats/hit-test-floats-005.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[hit-test-floats-005.html]
-  [Miss clipped float]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata-layout-2020/css/CSS2/linebox/inline-negative-margin-001.html.ini b/tests/wpt/metadata-layout-2020/css/CSS2/linebox/inline-negative-margin-001.html.ini
index 6425f4beda1..cb2edcb5dcd 100644
--- a/tests/wpt/metadata-layout-2020/css/CSS2/linebox/inline-negative-margin-001.html.ini
+++ b/tests/wpt/metadata-layout-2020/css/CSS2/linebox/inline-negative-margin-001.html.ini
@@ -5,9 +5,6 @@
   [[data-expected-height\] 7]
     expected: FAIL
 
-  [[data-expected-height\] 1]
-    expected: FAIL
-
-  [[data-expected-height\] 2]
+  [[data-expected-height\] 4]
     expected: FAIL
 
diff --git a/tests/wpt/metadata-layout-2020/css/cssom-view/elementFromPoint-001.html.ini b/tests/wpt/metadata-layout-2020/css/cssom-view/elementFromPoint-001.html.ini
new file mode 100644
index 00000000000..e38782d8c85
--- /dev/null
+++ b/tests/wpt/metadata-layout-2020/css/cssom-view/elementFromPoint-001.html.ini
@@ -0,0 +1,4 @@
+[elementFromPoint-001.html]
+  [CSSOM View - 5 - extensions to the Document interface]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata-layout-2020/fetch/content-type/response.window.js.ini b/tests/wpt/metadata-layout-2020/fetch/content-type/response.window.js.ini
index e24c600bd5a..c0265c5ad2a 100644
--- a/tests/wpt/metadata-layout-2020/fetch/content-type/response.window.js.ini
+++ b/tests/wpt/metadata-layout-2020/fetch/content-type/response.window.js.ini
@@ -312,24 +312,21 @@
   [Response: combined response Content-Type: text/html;" \\" text/plain ";charset=GBK]
     expected: NOTRUN
 
-  [<iframe>: separate response Content-Type: text/plain */*;charset=gbk]
-    expected: FAIL
-
   [<iframe>: combined response Content-Type: text/html;" text/plain]
     expected: FAIL
 
   [<iframe>: combined response Content-Type: text/html */*;charset=gbk]
     expected: FAIL
 
-  [<iframe>: combined response Content-Type: */* text/html]
-    expected: FAIL
-
-  [<iframe>: combined response Content-Type: text/html;charset=gbk text/plain text/html]
-    expected: FAIL
-
   [<iframe>: separate response Content-Type: text/plain */*]
     expected: FAIL
 
-  [<iframe>: combined response Content-Type: text/html;x=" text/plain]
+  [<iframe>: combined response Content-Type: text/html */*]
+    expected: FAIL
+
+  [<iframe>: separate response Content-Type: text/html;" text/plain]
+    expected: FAIL
+
+  [<iframe>: separate response Content-Type: text/html;" \\" text/plain]
     expected: FAIL
 
diff --git a/tests/wpt/metadata-layout-2020/fetch/nosniff/parsing-nosniff.window.js.ini b/tests/wpt/metadata-layout-2020/fetch/nosniff/parsing-nosniff.window.js.ini
index 30e1b851fd4..aa6eb8f6085 100644
--- a/tests/wpt/metadata-layout-2020/fetch/nosniff/parsing-nosniff.window.js.ini
+++ b/tests/wpt/metadata-layout-2020/fetch/nosniff/parsing-nosniff.window.js.ini
@@ -11,3 +11,6 @@
   [X-Content-Type-Options%3A%20nosniff%2C%2C%40%23%24%23%25%25%26%5E%26%5E*()()11!]
     expected: FAIL
 
+  [X-Content-Type-Options%3A%20'NosniFF']
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_4.html.ini b/tests/wpt/metadata-layout-2020/html/browsers/history/the-history-interface/traverse_the_history_3.html.ini
similarity index 71%
rename from tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_4.html.ini
rename to tests/wpt/metadata-layout-2020/html/browsers/history/the-history-interface/traverse_the_history_3.html.ini
index 385376c7321..51f8272a6de 100644
--- a/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_4.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/browsers/history/the-history-interface/traverse_the_history_3.html.ini
@@ -1,4 +1,4 @@
-[traverse_the_history_4.html]
+[traverse_the_history_3.html]
   [Multiple history traversals, last would be aborted]
     expected: FAIL
 
diff --git a/tests/wpt/metadata-layout-2020/html/browsers/origin/cross-origin-objects/cross-origin-objects-on-new-window.html.ini b/tests/wpt/metadata-layout-2020/html/browsers/origin/cross-origin-objects/cross-origin-objects-on-new-window.html.ini
deleted file mode 100644
index 735a9a75a2a..00000000000
--- a/tests/wpt/metadata-layout-2020/html/browsers/origin/cross-origin-objects/cross-origin-objects-on-new-window.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[cross-origin-objects-on-new-window.html]
-  expected: TIMEOUT
diff --git a/tests/wpt/metadata-layout-2020/html/browsers/windows/embedded-opener-remove-frame.html.ini b/tests/wpt/metadata-layout-2020/html/browsers/windows/embedded-opener-remove-frame.html.ini
index 9a85a905394..8e2a63e3b7f 100644
--- a/tests/wpt/metadata-layout-2020/html/browsers/windows/embedded-opener-remove-frame.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/browsers/windows/embedded-opener-remove-frame.html.ini
@@ -1,4 +1,5 @@
 [embedded-opener-remove-frame.html]
+  expected: CRASH
   [opener of discarded nested browsing context]
     expected: FAIL
 
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/media-elements/networkState_during_loadstart.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/media-elements/networkState_during_loadstart.html.ini
deleted file mode 100644
index 55a1023d419..00000000000
--- a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/media-elements/networkState_during_loadstart.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[networkState_during_loadstart.html]
-  expected: TIMEOUT
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini
index 88f4ddd0e9a..42ebcc9d57e 100644
--- a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini
@@ -1,5 +1,5 @@
 [iframe_sandbox_popups_escaping-1.html]
-  expected: CRASH
+  expected: TIMEOUT
   [Check that popups from a sandboxed iframe escape the sandbox if\n       allow-popups-to-escape-sandbox is used]
     expected: TIMEOUT
 
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini
index c6f45be1eb2..e63fe7c263e 100644
--- a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini
@@ -1,5 +1,5 @@
 [iframe_sandbox_popups_escaping-2.html]
-  expected: TIMEOUT
+  expected: CRASH
   [Check that popups from a sandboxed iframe escape the sandbox if\n       allow-popups-to-escape-sandbox is used]
     expected: TIMEOUT
 
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini
index 338d283eb0e..295031c1812 100644
--- a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini
@@ -1,4 +1,5 @@
 [iframe_sandbox_popups_escaping-3.html]
+  expected: TIMEOUT
   [Check that popups from a sandboxed iframe escape the sandbox if\n       allow-popups-to-escape-sandbox is used]
-    expected: FAIL
+    expected: TIMEOUT
 
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini
index 0407f0cc2b7..9eb581fcf1f 100644
--- a/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini
@@ -1,5 +1,5 @@
 [iframe_sandbox_popups_nonescaping-2.html]
-  expected: TIMEOUT
+  expected: CRASH
   [Check that popups from a sandboxed iframe do not escape the sandbox]
     expected: NOTRUN
 
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/forms/form-submission-0/form-double-submit-3.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/forms/form-submission-0/form-double-submit-3.html.ini
new file mode 100644
index 00000000000..9f416703229
--- /dev/null
+++ b/tests/wpt/metadata-layout-2020/html/semantics/forms/form-submission-0/form-double-submit-3.html.ini
@@ -0,0 +1,4 @@
+[form-double-submit-3.html]
+  [<button> should have the same double-submit protection as <input type=submit>]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata-layout-2020/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini b/tests/wpt/metadata-layout-2020/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini
index b1aa0f1f1c4..23fb39e544c 100644
--- a/tests/wpt/metadata-layout-2020/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini
@@ -1,13 +1,14 @@
 [htmlanchorelement_noopener.html]
+  expected: TIMEOUT
   [Check that targeting of rel=noopener with a given name reuses an existing window with that name]
     expected: FAIL
 
   [Check that rel=noopener with target=_top does a normal load]
-    expected: FAIL
+    expected: NOTRUN
 
   [Check that rel=noopener with target=_parent does a normal load]
-    expected: FAIL
+    expected: NOTRUN
 
   [Check that rel=noopener with target=_self does a normal load]
-    expected: FAIL
+    expected: NOTRUN
 
diff --git a/tests/wpt/metadata-layout-2020/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini b/tests/wpt/metadata-layout-2020/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini
index 5602ea31dbe..450c88bd32b 100644
--- a/tests/wpt/metadata-layout-2020/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini
@@ -4,5 +4,5 @@
     expected: TIMEOUT
 
   [The entry settings object while executing the compiled callback via Web IDL's invoke must be that of the node document]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata-layout-2020/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini b/tests/wpt/metadata-layout-2020/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini
index 16614b18649..d65177c2a5b 100644
--- a/tests/wpt/metadata-layout-2020/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini
+++ b/tests/wpt/metadata-layout-2020/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini
@@ -1,6 +1,7 @@
 [promise-rejection-events.html]
+  expected: TIMEOUT
   [delayed handling: delaying handling rejected promise created from createImageBitmap will cause both events to fire]
-    expected: FAIL
+    expected: TIMEOUT
 
   [unhandledrejection: from createImageBitmap which is UA triggered]
     expected: FAIL
diff --git a/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini b/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini
index 934f66fb058..a45bb55745f 100644
--- a/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini
+++ b/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini
@@ -161,3 +161,6 @@
   [X SNR (43.824040882296586 dB) is not greater than or equal to 65.737. Got 43.824040882296586.]
     expected: FAIL
 
+  [X Stitched sine-wave buffers at sample rate 43800 does not equal [0,0.06264832615852356,0.12505052983760834,0.18696144223213196,0.24813786149024963,0.308339387178421,0.36732959747314453,0.4248766601085663,0.480754554271698,0.5347436666488647,0.5866320133209229,0.6362156271934509,0.6832997798919678,0.7276994585990906,0.7692402601242065,0.8077589869499207...\] with an element-wise tolerance of {"absoluteThreshold":0.0038986,"relativeThreshold":0}.\n\tIndex\tActual\t\t\tExpected\t\tAbsError\t\tRelError\t\tTest threshold\n\t[28696\]\t2.9820817510119912e-20\t9.3139332532882690e-1\t9.3139332532882690e-1\t1.0000000000000000e+0\t3.8985999999999999e-3\n\t[28697\]\t7.0477002859115601e-1\t9.0675884485244751e-1\t2.0198881626129150e-1\t2.2275913536212616e-1\t3.8985999999999999e-3\n\tMax AbsError of 9.3139332532882690e-1 at index of 28696.\n\tMax RelError of 1.0000000000000000e+0 at index of 28696.\n]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini b/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini
index f1281b5ae81..b7e13cf7726 100644
--- a/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini
+++ b/tests/wpt/metadata-layout-2020/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini
@@ -1,5 +1,4 @@
 [audiocontext-not-fully-active.html]
-  expected: TIMEOUT
   [frame in navigated remote-site frame]
     expected: FAIL
 
diff --git a/tests/wpt/metadata-layout-2020/webmessaging/with-ports/018.html.ini b/tests/wpt/metadata-layout-2020/webmessaging/with-ports/018.html.ini
new file mode 100644
index 00000000000..663a1f8fa30
--- /dev/null
+++ b/tests/wpt/metadata-layout-2020/webmessaging/with-ports/018.html.ini
@@ -0,0 +1,5 @@
+[018.html]
+  expected: TIMEOUT
+  [origin of the script that invoked the method, javascript:]
+    expected: TIMEOUT
+
diff --git a/tests/wpt/metadata-layout-2020/webmessaging/without-ports/018.html.ini b/tests/wpt/metadata-layout-2020/webmessaging/without-ports/018.html.ini
new file mode 100644
index 00000000000..663a1f8fa30
--- /dev/null
+++ b/tests/wpt/metadata-layout-2020/webmessaging/without-ports/018.html.ini
@@ -0,0 +1,5 @@
+[018.html]
+  expected: TIMEOUT
+  [origin of the script that invoked the method, javascript:]
+    expected: TIMEOUT
+
diff --git a/tests/wpt/metadata-layout-2020/workers/semantics/run-a-worker/003.html.ini b/tests/wpt/metadata-layout-2020/workers/semantics/run-a-worker/003.html.ini
index c2c57aec558..dd49aa8e457 100644
--- a/tests/wpt/metadata-layout-2020/workers/semantics/run-a-worker/003.html.ini
+++ b/tests/wpt/metadata-layout-2020/workers/semantics/run-a-worker/003.html.ini
@@ -1,4 +1,5 @@
 [003.html]
+  expected: ERROR
   [shared]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index 49745f83e85..1fd3e640d66 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -329883,7 +329883,7 @@
      []
     ],
     "density-corrected-size-bg-ref.html": [
-     "758ca8edc60adccf491f7db3aa40dbba9d75798b",
+     "154c1344444c45180628d858ed053434fbd9cab2",
      []
     ],
     "density-corrected-size-img-ref.html": [
@@ -347221,17 +347221,29 @@
       "6092c0d767050d0c1b13cfd2360f7a7f17d656a5",
       []
      ],
+     "common-test-service-worker.js": [
+      "6985c901fa683a2585ae28cdb3cd5a9711df8222",
+      []
+     ],
      "resources": {
+      "common-test-helper-iframe.js": [
+       "597955fea1f9dc115b6910df59fbb0c5edc1d008",
+       []
+      ],
       "common-test-helper.js": [
-       "f7269dba06743efec3bc9aae21556df66d09f52a",
+       "2bf99184a0c9c944551b472a3be6960dbf159128",
        []
       ],
       "data-base-url.json": [
        "81fcf087425c70eefa7d3c3465be59684fb1a521",
        []
       ],
+      "empty-import-map-internal.json": [
+       "59390c89f6b4da9ca95da0142d48022e8dc25f40",
+       []
+      ],
       "empty-import-map.json": [
-       "ce6c185498fa38c5e2fcd4e7ba36503778ad0ba7",
+       "ff85a6d69d5ded8d4fb917ae767748d8903bc4e7",
        []
       ],
       "overlapping-entries.json": [
@@ -348758,7 +348770,7 @@
     ]
    },
    "lint.ignore": [
-    "fe2933bb8a31cb7d5a8c39eee06626f1c7b261ac",
+    "b44faa491f9ad0b4873b9d56cff479310e9c392c",
     []
    ],
    "loading": {
@@ -349738,7 +349750,7 @@
       []
      ],
      "mediasource-worker-util.js": [
-      "e0c4bdc0e10bb5707f7af75bf6cafa293a6f7b9b",
+      "4cee4862fa4e6c39c43fee7af1a332e8a7cd7ce2",
       []
      ]
     },
@@ -469913,6 +469925,13 @@
          {}
         ]
        ],
+       "fieldset-percent-height.html": [
+        "bbe40785be2f0738896e648ce23bc90b9f50c558",
+        [
+         null,
+         {}
+        ]
+       ],
        "fieldset-percentage-block-size.html": [
         "1c221555c305bd9039558389b3f22ee00d65f8b9",
         [
@@ -484958,8 +484977,8 @@
      }
     },
     "common": {
-     "parsing.tentative.html": [
-      "d3334d7bbb9b4406397f96b596e71463eb252791",
+     "parsing.tentative.https.html": [
+      "0dc9e24c8d84ddf06ef38f874605a20712cda214",
       [
        null,
        {
@@ -484967,8 +484986,17 @@
        }
       ]
      ],
-     "resolving.tentative.html": [
-      "8dd3e3a2e710f52b05cc34e36a8535d211a480de",
+     "resolving-internal.tentative.https.html": [
+      "a9f7c82fd879ca557afd2db85d35901e066e919b",
+      [
+       null,
+       {
+        "timeout": "long"
+       }
+      ]
+     ],
+     "resolving.tentative.https.html": [
+      "abb30e0afa4f41c306d2a5b1eee788bdd94390bb",
       [
        null,
        {
@@ -489208,15 +489236,22 @@
      ]
     ],
     "dedicated-worker": {
-     "mediasource-worker-attach.html": [
-      "b09f05c248531029b9b787597af1ae84e0b65ccd",
+     "mediasource-worker-objecturl.html": [
+      "382a9a4f3632a53cd2a94dab121fe34a07d9238d",
       [
        null,
        {}
       ]
      ],
-     "mediasource-worker-objecturl.html": [
-      "382a9a4f3632a53cd2a94dab121fe34a07d9238d",
+     "mediasource-worker-play-terminate-worker.html": [
+      "d1c1c3d3fe8b0feba9eebb8c4796e904c0bcbf2f",
+      [
+       null,
+       {}
+      ]
+     ],
+     "mediasource-worker-play.html": [
+      "200f8a851bfc396861ad93b501bcebd7209dffd6",
       [
        null,
        {}
@@ -511526,7 +511561,7 @@
      ]
     ],
     "element-based-offset-clamp.html": [
-     "6467af199f10224e16f07badf16b73593270f14c",
+     "b140507493848688f38021573643ce7d6321e514",
      [
       null,
       {}
diff --git a/tests/wpt/metadata/css/CSS2/floats/hit-test-floats-002.html.ini b/tests/wpt/metadata/css/CSS2/floats/hit-test-floats-002.html.ini
deleted file mode 100644
index f64b45fea6b..00000000000
--- a/tests/wpt/metadata/css/CSS2/floats/hit-test-floats-002.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[hit-test-floats-002.html]
-  [Hit test float]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/CSS2/floats/hit-test-floats-005.html.ini b/tests/wpt/metadata/css/CSS2/floats/hit-test-floats-005.html.ini
deleted file mode 100644
index baa9f1a7541..00000000000
--- a/tests/wpt/metadata/css/CSS2/floats/hit-test-floats-005.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[hit-test-floats-005.html]
-  [Miss clipped float]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/CSS2/linebox/inline-negative-margin-001.html.ini b/tests/wpt/metadata/css/CSS2/linebox/inline-negative-margin-001.html.ini
index 076c791cd3d..94a3570d26b 100644
--- a/tests/wpt/metadata/css/CSS2/linebox/inline-negative-margin-001.html.ini
+++ b/tests/wpt/metadata/css/CSS2/linebox/inline-negative-margin-001.html.ini
@@ -8,9 +8,6 @@
   [[data-expected-height\] 3]
     expected: FAIL
 
-  [[data-expected-height\] 1]
-    expected: FAIL
-
-  [[data-expected-height\] 2]
+  [[data-expected-height\] 4]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/css/cssom-view/elementFromPoint-001.html.ini b/tests/wpt/metadata/css/cssom-view/elementFromPoint-001.html.ini
new file mode 100644
index 00000000000..e38782d8c85
--- /dev/null
+++ b/tests/wpt/metadata/css/cssom-view/elementFromPoint-001.html.ini
@@ -0,0 +1,4 @@
+[elementFromPoint-001.html]
+  [CSSOM View - 5 - extensions to the Document interface]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/fetch/content-type/response.window.js.ini b/tests/wpt/metadata/fetch/content-type/response.window.js.ini
index f00d81ae57f..fb61a2ffc71 100644
--- a/tests/wpt/metadata/fetch/content-type/response.window.js.ini
+++ b/tests/wpt/metadata/fetch/content-type/response.window.js.ini
@@ -312,24 +312,21 @@
   [fetch(): separate response Content-Type: text/plain ]
     expected: NOTRUN
 
-  [<iframe>: separate response Content-Type: text/plain */*;charset=gbk]
-    expected: FAIL
-
   [<iframe>: combined response Content-Type: text/html;" text/plain]
     expected: FAIL
 
   [<iframe>: combined response Content-Type: text/html */*;charset=gbk]
     expected: FAIL
 
-  [<iframe>: combined response Content-Type: */* text/html]
-    expected: FAIL
-
-  [<iframe>: combined response Content-Type: text/html;charset=gbk text/plain text/html]
-    expected: FAIL
-
   [<iframe>: separate response Content-Type: text/plain */*]
     expected: FAIL
 
-  [<iframe>: combined response Content-Type: text/html;x=" text/plain]
+  [<iframe>: combined response Content-Type: text/html */*]
+    expected: FAIL
+
+  [<iframe>: separate response Content-Type: text/html;" text/plain]
+    expected: FAIL
+
+  [<iframe>: separate response Content-Type: text/html;" \\" text/plain]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/fetch/nosniff/parsing-nosniff.window.js.ini b/tests/wpt/metadata/fetch/nosniff/parsing-nosniff.window.js.ini
index 30e1b851fd4..aa6eb8f6085 100644
--- a/tests/wpt/metadata/fetch/nosniff/parsing-nosniff.window.js.ini
+++ b/tests/wpt/metadata/fetch/nosniff/parsing-nosniff.window.js.ini
@@ -11,3 +11,6 @@
   [X-Content-Type-Options%3A%20nosniff%2C%2C%40%23%24%23%25%25%26%5E%26%5E*()()11!]
     expected: FAIL
 
+  [X-Content-Type-Options%3A%20'NosniFF']
+    expected: FAIL
+
diff --git a/tests/wpt/metadata-layout-2020/html/browsers/history/the-history-interface/traverse_the_history_4.html.ini b/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_3.html.ini
similarity index 71%
rename from tests/wpt/metadata-layout-2020/html/browsers/history/the-history-interface/traverse_the_history_4.html.ini
rename to tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_3.html.ini
index 385376c7321..51f8272a6de 100644
--- a/tests/wpt/metadata-layout-2020/html/browsers/history/the-history-interface/traverse_the_history_4.html.ini
+++ b/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_3.html.ini
@@ -1,4 +1,4 @@
-[traverse_the_history_4.html]
+[traverse_the_history_3.html]
   [Multiple history traversals, last would be aborted]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/html/browsers/origin/cross-origin-objects/cross-origin-objects-on-new-window.html.ini b/tests/wpt/metadata/html/browsers/origin/cross-origin-objects/cross-origin-objects-on-new-window.html.ini
deleted file mode 100644
index 735a9a75a2a..00000000000
--- a/tests/wpt/metadata/html/browsers/origin/cross-origin-objects/cross-origin-objects-on-new-window.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[cross-origin-objects-on-new-window.html]
-  expected: TIMEOUT
diff --git a/tests/wpt/metadata/html/browsers/windows/embedded-opener-remove-frame.html.ini b/tests/wpt/metadata/html/browsers/windows/embedded-opener-remove-frame.html.ini
index 442db3cb549..2532dceabac 100644
--- a/tests/wpt/metadata/html/browsers/windows/embedded-opener-remove-frame.html.ini
+++ b/tests/wpt/metadata/html/browsers/windows/embedded-opener-remove-frame.html.ini
@@ -1,4 +1,5 @@
 [embedded-opener-remove-frame.html]
+  expected: CRASH
   [opener and "removed" embedded documents]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/media-elements/networkState_during_loadstart.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/media-elements/networkState_during_loadstart.html.ini
deleted file mode 100644
index 55a1023d419..00000000000
--- a/tests/wpt/metadata/html/semantics/embedded-content/media-elements/networkState_during_loadstart.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[networkState_during_loadstart.html]
-  expected: TIMEOUT
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini
index 2a166bb97b7..fc37df7e3fa 100644
--- a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini
+++ b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-1.html.ini
@@ -1,6 +1,6 @@
 [iframe_sandbox_popups_escaping-1.html]
   type: testharness
-  expected: CRASH
+  expected: TIMEOUT
   [Check that popups from a sandboxed iframe escape the sandbox if\n       allow-popups-to-escape-sandbox is used]
     expected: TIMEOUT
 
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini
index c6f45be1eb2..e63fe7c263e 100644
--- a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini
+++ b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-2.html.ini
@@ -1,5 +1,5 @@
 [iframe_sandbox_popups_escaping-2.html]
-  expected: TIMEOUT
+  expected: CRASH
   [Check that popups from a sandboxed iframe escape the sandbox if\n       allow-popups-to-escape-sandbox is used]
     expected: TIMEOUT
 
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini
index f6a7aca3306..5f60c78e73c 100644
--- a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini
+++ b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_escaping-3.html.ini
@@ -1,5 +1,6 @@
 [iframe_sandbox_popups_escaping-3.html]
   type: testharness
+  expected: TIMEOUT
   [Check that popups from a sandboxed iframe escape the sandbox if\n       allow-popups-to-escape-sandbox is used]
-    expected: FAIL
+    expected: TIMEOUT
 
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini
index d43f38b40cd..45d8be1c898 100644
--- a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini
+++ b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-2.html.ini
@@ -1,6 +1,6 @@
 [iframe_sandbox_popups_nonescaping-2.html]
   type: testharness
-  expected: TIMEOUT
+  expected: CRASH
   [Check that popups from a sandboxed iframe do not escape the sandbox]
     expected: NOTRUN
 
diff --git a/tests/wpt/metadata/html/semantics/forms/form-submission-0/form-double-submit-3.html.ini b/tests/wpt/metadata/html/semantics/forms/form-submission-0/form-double-submit-3.html.ini
new file mode 100644
index 00000000000..9f416703229
--- /dev/null
+++ b/tests/wpt/metadata/html/semantics/forms/form-submission-0/form-double-submit-3.html.ini
@@ -0,0 +1,4 @@
+[form-double-submit-3.html]
+  [<button> should have the same double-submit protection as <input type=submit>]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini b/tests/wpt/metadata/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini
index a057142bbc5..510ea619a7d 100644
--- a/tests/wpt/metadata/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini
+++ b/tests/wpt/metadata/html/semantics/links/links-created-by-a-and-area-elements/htmlanchorelement_noopener.html.ini
@@ -1,5 +1,6 @@
 [htmlanchorelement_noopener.html]
   type: testharness
+  expected: TIMEOUT
   [Check that targeting of rel=noopener with a given name ignores an existing window with that name]
     expected: NOTRUN
 
@@ -7,11 +8,11 @@
     expected: FAIL
 
   [Check that rel=noopener with target=_top does a normal load]
-    expected: FAIL
+    expected: NOTRUN
 
   [Check that rel=noopener with target=_parent does a normal load]
-    expected: FAIL
+    expected: NOTRUN
 
   [Check that rel=noopener with target=_self does a normal load]
-    expected: FAIL
+    expected: NOTRUN
 
diff --git a/tests/wpt/metadata/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini b/tests/wpt/metadata/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini
index edae5e5adcd..01f7b72cbe7 100644
--- a/tests/wpt/metadata/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini
+++ b/tests/wpt/metadata/html/webappapis/scripting/events/compile-event-handler-settings-objects.html.ini
@@ -5,5 +5,5 @@
     expected: TIMEOUT
 
   [The entry settings object while executing the compiled callback via Web IDL's invoke must be that of the node document]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini b/tests/wpt/metadata/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini
index 16614b18649..d65177c2a5b 100644
--- a/tests/wpt/metadata/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini
+++ b/tests/wpt/metadata/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events.html.ini
@@ -1,6 +1,7 @@
 [promise-rejection-events.html]
+  expected: TIMEOUT
   [delayed handling: delaying handling rejected promise created from createImageBitmap will cause both events to fire]
-    expected: FAIL
+    expected: TIMEOUT
 
   [unhandledrejection: from createImageBitmap which is UA triggered]
     expected: FAIL
diff --git a/tests/wpt/metadata/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini b/tests/wpt/metadata/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini
index 0dc0fb65e8f..a944178a615 100644
--- a/tests/wpt/metadata/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini
+++ b/tests/wpt/metadata/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html.ini
@@ -389,3 +389,6 @@
   [X SNR (43.824040882296586 dB) is not greater than or equal to 65.737. Got 43.824040882296586.]
     expected: FAIL
 
+  [X Stitched sine-wave buffers at sample rate 43800 does not equal [0,0.06264832615852356,0.12505052983760834,0.18696144223213196,0.24813786149024963,0.308339387178421,0.36732959747314453,0.4248766601085663,0.480754554271698,0.5347436666488647,0.5866320133209229,0.6362156271934509,0.6832997798919678,0.7276994585990906,0.7692402601242065,0.8077589869499207...\] with an element-wise tolerance of {"absoluteThreshold":0.0038986,"relativeThreshold":0}.\n\tIndex\tActual\t\t\tExpected\t\tAbsError\t\tRelError\t\tTest threshold\n\t[28696\]\t2.9820817510119912e-20\t9.3139332532882690e-1\t9.3139332532882690e-1\t1.0000000000000000e+0\t3.8985999999999999e-3\n\t[28697\]\t7.0477002859115601e-1\t9.0675884485244751e-1\t2.0198881626129150e-1\t2.2275913536212616e-1\t3.8985999999999999e-3\n\tMax AbsError of 9.3139332532882690e-1 at index of 28696.\n\tMax RelError of 1.0000000000000000e+0 at index of 28696.\n]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini b/tests/wpt/metadata/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini
index f1281b5ae81..b7e13cf7726 100644
--- a/tests/wpt/metadata/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini
+++ b/tests/wpt/metadata/webaudio/the-audio-api/the-audiocontext-interface/audiocontext-not-fully-active.html.ini
@@ -1,5 +1,4 @@
 [audiocontext-not-fully-active.html]
-  expected: TIMEOUT
   [frame in navigated remote-site frame]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/webmessaging/with-ports/018.html.ini b/tests/wpt/metadata/webmessaging/with-ports/018.html.ini
new file mode 100644
index 00000000000..663a1f8fa30
--- /dev/null
+++ b/tests/wpt/metadata/webmessaging/with-ports/018.html.ini
@@ -0,0 +1,5 @@
+[018.html]
+  expected: TIMEOUT
+  [origin of the script that invoked the method, javascript:]
+    expected: TIMEOUT
+
diff --git a/tests/wpt/metadata/webmessaging/without-ports/018.html.ini b/tests/wpt/metadata/webmessaging/without-ports/018.html.ini
new file mode 100644
index 00000000000..663a1f8fa30
--- /dev/null
+++ b/tests/wpt/metadata/webmessaging/without-ports/018.html.ini
@@ -0,0 +1,5 @@
+[018.html]
+  expected: TIMEOUT
+  [origin of the script that invoked the method, javascript:]
+    expected: TIMEOUT
+
diff --git a/tests/wpt/metadata/workers/semantics/run-a-worker/003.html.ini b/tests/wpt/metadata/workers/semantics/run-a-worker/003.html.ini
index d6e39444229..49f489d2a1c 100644
--- a/tests/wpt/metadata/workers/semantics/run-a-worker/003.html.ini
+++ b/tests/wpt/metadata/workers/semantics/run-a-worker/003.html.ini
@@ -1,5 +1,6 @@
 [003.html]
   type: testharness
+  expected: ERROR
   [shared]
     expected: FAIL
 
diff --git a/tests/wpt/web-platform-tests/density-size-correction/density-corrected-size-bg-ref.html b/tests/wpt/web-platform-tests/density-size-correction/density-corrected-size-bg-ref.html
index 758ca8edc60..154c1344444 100644
--- a/tests/wpt/web-platform-tests/density-size-correction/density-corrected-size-bg-ref.html
+++ b/tests/wpt/web-platform-tests/density-size-correction/density-corrected-size-bg-ref.html
@@ -6,26 +6,24 @@
 <body>
     <style>
         body {
-            --lores: url(resources/exif-resolution-valid-lores.jpg);
-            --hires: url(resources/exif-resolution-valid-hires.jpg);
             --default: url(resources/exif-resolution-none.jpg);
-            --non-uniform: url(resources/exif-resolution-valid-non-uniform.jpg);
         }
         .default-bg {background-image: var(--default); }
-        .lores-bg {background-image: var(--lores); }
-        .hires-bg {background-image: var(--hires); }
+        .lores-bg {background-image: var(--default); background-size: 200px 100px; }
+        .hires-bg {background-image: var(--default); background-size: 50px 25px; }
         .invalid-bg {background-image: var(--invalid); }
-        .non-uniform-bg {background-image: var(--non-uniform); }
+        .non-uniform-bg {background-image: var(--default); background-size: 50px 100px; }
         .box { width: 200px; height: 200px; display: inline-block; }
         .tiled {background-repeat: repeat; }
         .stretch {background-repeat: no-repeat; background-size: contain; }
+        .non-uniform-bg.stretch { background-size: 100px 200px; }
     </style>
     <div class="default-bg tiled box"></div>
     <div class="lores-bg tiled box"></div>
     <div class="hires-bg tiled box"></div>
     <div class="non-uniform-bg tiled box"></div>
     <br/>
-    <div class="lores-bg stretch box"></div>
+    <div class="default-bg stretch box"></div>
     <div class="default-bg stretch box"></div>
     <div class="non-uniform-bg stretch box"></div>
 </body>
diff --git a/tests/wpt/web-platform-tests/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/fieldset-percent-height.html b/tests/wpt/web-platform-tests/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/fieldset-percent-height.html
new file mode 100644
index 00000000000..bbe40785be2
--- /dev/null
+++ b/tests/wpt/web-platform-tests/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/fieldset-percent-height.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<table cellspacing="0" cellpadding="0" style="width:100px; height:60px;">
+  <tr>
+    <td>
+      <fieldset style="border:none; padding:0; height:100%; margin:0; margin-top:13px;">
+        <div><div id="elm"></div></div>
+      </fieldset>
+    </td>
+  </tr>
+</table>
+<script>
+// crbug.com/1138204. Though the specification doesn't mention this behavior,
+// there must be no doubt about the expected behavior.
+test(() => {
+  const fieldset = document.querySelector('fieldset');
+  const initialHeight = fieldset.offsetHeight;
+  document.querySelector('#elm').style.display = 'none';
+  assert_equals(fieldset.offsetHeight, initialHeight);
+}, 'Fieldset with a percent height should not increase the height on every reflow.');
+</script>
diff --git a/tests/wpt/web-platform-tests/import-maps/common/common-test-service-worker.js b/tests/wpt/web-platform-tests/import-maps/common/common-test-service-worker.js
new file mode 100644
index 00000000000..6985c901fa6
--- /dev/null
+++ b/tests/wpt/web-platform-tests/import-maps/common/common-test-service-worker.js
@@ -0,0 +1,28 @@
+let serveImporterScript = false;
+
+self.addEventListener('message', event => {
+  serveImporterScript = true;
+  event.source.postMessage('Done');
+});
+
+self.addEventListener('fetch', event => {
+    if (event.request.url.indexOf('common-test-helper-iframe.js') >= 0) {
+      return;
+    }
+    if (serveImporterScript) {
+      serveImporterScript = false;
+      event.respondWith(
+        new Response(
+          'window.importHelper = (specifier) => import(specifier);',
+          {headers: {'Content-Type': 'text/javascript'}}
+        ));
+    } else {
+      event.respondWith(
+        new Response(
+          'export const response = ' +
+              JSON.stringify({url: event.request.url}) + ';',
+          {headers: {'Access-Control-Allow-Origin': '*',
+                     'Content-Type': 'text/javascript'}}
+        ));
+    }
+});
diff --git a/tests/wpt/web-platform-tests/import-maps/common/parsing.tentative.html b/tests/wpt/web-platform-tests/import-maps/common/parsing.tentative.https.html
similarity index 61%
rename from tests/wpt/web-platform-tests/import-maps/common/parsing.tentative.html
rename to tests/wpt/web-platform-tests/import-maps/common/parsing.tentative.https.html
index d3334d7bbb9..0dc9e24c8d8 100644
--- a/tests/wpt/web-platform-tests/import-maps/common/parsing.tentative.html
+++ b/tests/wpt/web-platform-tests/import-maps/common/parsing.tentative.https.html
@@ -2,9 +2,16 @@
 <meta name="timeout" content="long">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
+<script src="/service-workers/service-worker/resources/test-helpers.sub.js"></script>
+<script>
+// All parsing tests requires Chromium's internal methods.
+globalThis.useInternalMethods = true;
+</script>
 <body>
 <script type="module">
-import { runTestsFromJSON } from "./resources/common-test-helper.js";
+import { runTestsFromJSON, setupGlobalCleanup } from "./resources/common-test-helper.js";
+
+const promises = [];
 
 for (const json of [
   'resources/parsing-addresses-absolute.json',
@@ -19,8 +26,13 @@ for (const json of [
   'resources/parsing-specifier-keys.json',
   'resources/parsing-trailing-slashes.json',
 ]) {
-  promise_test(() =>
-    runTestsFromJSON(json),
+  promise_test(() => {
+      const promise = runTestsFromJSON(json);
+      promises.push(promise);
+      return promise;
+    },
     "Test helper: fetching and sanity checking test JSON: " + json);
 }
+
+Promise.all(promises).then(setupGlobalCleanup);
 </script>
diff --git a/tests/wpt/web-platform-tests/import-maps/common/resolving-internal.tentative.https.html b/tests/wpt/web-platform-tests/import-maps/common/resolving-internal.tentative.https.html
new file mode 100644
index 00000000000..a9f7c82fd87
--- /dev/null
+++ b/tests/wpt/web-platform-tests/import-maps/common/resolving-internal.tentative.https.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<meta name="timeout" content="long">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/service-workers/service-worker/resources/test-helpers.sub.js"></script>
+<script>
+// This test file is for resolution tests that require Chromium's internal
+// methods.
+// For tests that don't use Chromium's internal methods, see
+// resolving.tentative.https.html.
+globalThis.useInternalMethods = true;
+</script>
+<body>
+<script type="module">
+import { runTestsFromJSON, setupGlobalCleanup } from "./resources/common-test-helper.js";
+
+const promises = [];
+
+for (const json of [
+  'resources/empty-import-map-internal.json',
+]) {
+  promise_test(() => {
+      const promise = runTestsFromJSON(json);
+      promises.push(promise);
+      return promise;
+    },
+    "Test helper: fetching and sanity checking test JSON: " + json);
+}
+
+Promise.all(promises).then(setupGlobalCleanup);
+</script>
diff --git a/tests/wpt/web-platform-tests/import-maps/common/resolving.tentative.html b/tests/wpt/web-platform-tests/import-maps/common/resolving.tentative.https.html
similarity index 62%
rename from tests/wpt/web-platform-tests/import-maps/common/resolving.tentative.html
rename to tests/wpt/web-platform-tests/import-maps/common/resolving.tentative.https.html
index 8dd3e3a2e71..abb30e0afa4 100644
--- a/tests/wpt/web-platform-tests/import-maps/common/resolving.tentative.html
+++ b/tests/wpt/web-platform-tests/import-maps/common/resolving.tentative.https.html
@@ -2,9 +2,12 @@
 <meta name="timeout" content="long">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
+<script src="/service-workers/service-worker/resources/test-helpers.sub.js"></script>
 <body>
 <script type="module">
-import { runTestsFromJSON } from "./resources/common-test-helper.js";
+import { runTestsFromJSON, setupGlobalCleanup } from "./resources/common-test-helper.js";
+
+const promises = [];
 
 for (const json of [
   'resources/scopes.json',
@@ -17,8 +20,13 @@ for (const json of [
   'resources/overlapping-entries.json',
   'resources/resolving-null.json',
 ]) {
-  promise_test(() =>
-    runTestsFromJSON(json),
+  promise_test(() => {
+      const promise = runTestsFromJSON(json);
+      promises.push(promise);
+      return promise;
+    },
     "Test helper: fetching and sanity checking test JSON: " + json);
 }
+
+Promise.all(promises).then(setupGlobalCleanup);
 </script>
diff --git a/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper-iframe.js b/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper-iframe.js
new file mode 100644
index 00000000000..597955fea1f
--- /dev/null
+++ b/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper-iframe.js
@@ -0,0 +1,103 @@
+// Handle errors around fetching, parsing and registering import maps.
+const onScriptError = event => {
+  window.registrationResult = {type: 'FetchError', error: event.error};
+  return false;
+};
+window.windowErrorHandler = event => {
+  window.registrationResult = {type: 'ParseError', error: event.error};
+  return false;
+};
+window.addEventListener('error', window.windowErrorHandler);
+
+// Handle specifier resolution requests from the parent frame.
+// For failures, we post error names and messages instead of error
+// objects themselves and re-create error objects later, to avoid
+// issues around serializing error objects which is a quite new feature.
+window.addEventListener('message', event => {
+  if (event.data.action === 'prepareResolve') {
+    // To get the result of #resolve-a-module-specifier given a script
+    // (with base URL = |baseURL|) and |specifier|, the service worker
+    // first serves an importer script with response URL = |baseURL|:
+    //     window.importHelper = (specifier) => import(specifier);
+    // This is to use |baseURL| as the referringScript's base URL.
+
+    // Step 1. Signal the service worker to serve
+    // the importer script for the next fetch request.
+    parent.worker.postMessage('serveImporterScript');
+  } else if (event.data.action === 'resolve') {
+    if (event.data.expectedURL === null ||
+        new URL(event.data.expectedURL).protocol === 'https:') {
+      // Testing without internal methods:
+      // If the resolution is expected to fail (null case here),
+      // we can test the failure just by catching the exception.
+      // If the expected URL is HTTPS, we can test the result by
+      // intercepting requests by service workers.
+
+      // Step 3. Evaluate the importer script as a classic script,
+      // in order to prevent |baseURL| from being mapped by import maps.
+      const script = document.createElement('script');
+      script.onload = () => {
+        // Step 4. Trigger dynamic import from |baseURL|.
+        importHelper(event.data.specifier)
+          .then(module => {
+              // Step 5. Service worker responds with a JSON containing
+              // the request URL for the dynamic import
+              // (= the result of #resolve-a-module-specifier).
+              parent.postMessage({type: 'ResolutionSuccess',
+                                  result: module.response.url},
+                                 '*');
+            })
+          .catch(e => {
+              parent.postMessage(
+                  {type: 'Failure', result: e.name, message: e.message},
+                  '*');
+            });
+      };
+      script.src = event.data.baseURL;
+      document.body.appendChild(script);
+    } else {
+      // Testing with internal methods.
+      // For example, the resolution results are data: URLs.
+      if (!event.data.useInternalMethods) {
+        parent.postMessage(
+            {type: 'Failure',
+             result: 'Error',
+             message: 'internals.resolveModuleSpecifier is not available'},
+            '*');
+        return;
+      }
+      try {
+        const result = internals.resolveModuleSpecifier(
+          event.data.specifier,
+          event.data.baseURL,
+          document);
+        parent.postMessage(
+            {type: 'ResolutionSuccess', result: result}, '*');
+      } catch (e) {
+        parent.postMessage(
+            {type: 'Failure', result: e.name, message: e.message}, '*');
+      }
+    }
+  } else if (event.data.action === 'getParsedImportMap') {
+    if (!event.data.useInternalMethods) {
+      parent.postMessage(
+          {type: 'Failure',
+           result: 'Error',
+           message: 'internals.getParsedImportMap is not available'},
+          '*');
+    }
+    try {
+      parent.postMessage({
+          type: 'GetParsedImportMapSuccess',
+          result: internals.getParsedImportMap(document)}, '*');
+    } catch (e) {
+      parent.postMessage(
+          {type: 'Failure', result: e.name, message: e.message}, '*');
+    }
+  } else {
+    parent.postMessage({
+        type: 'Failure',
+        result: 'Error',
+        message: 'Invalid Action: ' + event.data.action}, '*');
+  }
+});
diff --git a/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper.js b/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper.js
index f7269dba067..2bf99184a0c 100644
--- a/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper.js
+++ b/tests/wpt/web-platform-tests/import-maps/common/resources/common-test-helper.js
@@ -1,5 +1,27 @@
 setup({allow_uncaught_exception : true});
 
+// Set window.useInternalMethods = true when needed && available.
+
+let registration;
+const scope = './scope/';
+
+// Global setup: this must be the first promise_test.
+promise_test(async (t) => {
+  const script = 'common-test-service-worker.js';
+
+  registration =
+      await service_worker_unregister_and_register(t, script, scope);
+  window.worker = registration.installing;
+  await wait_for_state(t, window.worker, 'activated');
+}, 'global setup');
+
+export function setupGlobalCleanup() {
+  // Global cleanup: the final promise_test.
+  promise_test(() => {
+     return registration.unregister();
+  }, 'global cleanup');
+}
+
 // Creates a new Document (via <iframe>) and add an inline import map.
 function parse(importMap, importMapBaseURL) {
   return new Promise(resolve => {
@@ -15,79 +37,51 @@ function parse(importMap, importMapBaseURL) {
       {once: true});
 
     const testHTML = `
-      <script>
-      // Handle errors around fetching, parsing and registering import maps.
-      let registrationResult;
-      const onScriptError = event => {
-        registrationResult = {type: 'FetchError', error: event.error};
-        return false;
-      };
-      const windowErrorHandler = event => {
-        registrationResult = {type: 'ParseError', error: event.error};
-        return false;
-      };
-      window.addEventListener('error', windowErrorHandler);
-      window.addEventListener('load', event => {
-        if (!registrationResult) {
-          registrationResult = {type: 'Success'};
-        }
-        window.removeEventListener('error', windowErrorHandler);
-        parent.postMessage(registrationResult, '*');
-      });
-
-      // Handle specifier resolution requests from the parent frame.
-      window.addEventListener('message', event => {
-        try {
-          if (event.data.action === 'resolve') {
-            // URL resolution is tested using Chromium's internals.
-            // TODO(hiroshige): Remove the Chromium-specific dependency.
-            const result = internals.resolveModuleSpecifier(
-                event.data.specifier,
-                event.data.baseURL,
-                document);
-            parent.postMessage({type: 'ResolutionSuccess', result: result}, '*');
-         } else if (event.data.action === 'getParsedImportMap') {
-           parent.postMessage({
-               type: 'GetParsedImportMapSuccess',
-               result: internals.getParsedImportMap(document)}, '*');
-         } else {
-           parent.postMessage({
-               type: 'Failure',
-               result: "Invalid Action: " + event.data.action}, '*');
-         }
-        } catch (e) {
-          // We post error names instead of error objects themselves and
-          // re-create error objects later, to avoid issues around serializing
-          // error objects which is a quite new feature.
-          parent.postMessage({type: 'Failure', result: e.name}, '*');
-        }
-      });
-      </script>
+      <body>
+      <script src="${location.origin}/import-maps/common/resources/common-test-helper-iframe.js"></script>
       <script type="importmap" onerror="onScriptError(event)">
       ${importMapString}
       </script>
+      <script type="module">
+        if (!window.registrationResult) {
+          window.registrationResult = {type: 'Success'};
+        }
+        window.removeEventListener('error', window.windowErrorHandler);
+        parent.postMessage(window.registrationResult, '*');
+      </script>
+      </body>
     `;
 
     if (new URL(importMapBaseURL).protocol === 'data:') {
+      if (!window.useInternalMethods) {
+        throw new Error(
+            'Import maps with base URL = data: URL requires internal methods');
+      }
       iframe.src = 'data:text/html;base64,' + btoa(testHTML);
     } else {
-      iframe.srcdoc = `<base href="${importMapBaseURL}">` + testHTML;
+      // Set the src to `scope` in order to make requests from `iframe`
+      // intercepted by the service worker.
+      iframe.src = scope;
+      iframe.onload = () => {
+        iframe.contentDocument.write(
+            `<base href="${importMapBaseURL}">` + testHTML);
+        iframe.contentDocument.close();
+      };
     }
-
     document.body.appendChild(iframe);
-
   });
 }
 
 // Returns a promise that is resolved with the resulting URL.
-function resolve(specifier, parsedImportMap, baseURL) {
+// `expectedURL` is a string, or null if to be thrown.
+function resolve(specifier, parsedImportMap, baseURL, expectedURL) {
   return new Promise((resolve, reject) => {
     window.addEventListener('message', event => {
         if (event.data.type === 'ResolutionSuccess') {
           resolve(event.data.result);
         } else if (event.data.type === 'Failure') {
           if (event.data.result === 'TypeError') {
-            reject(new TypeError());
+            reject(new TypeError(event.data.message));
           } else {
             reject(new Error(event.data.result));
           }
@@ -97,8 +91,20 @@ function resolve(specifier, parsedImportMap, baseURL) {
       },
       {once: true});
 
-    parsedImportMap.contentWindow.postMessage(
-        {action: "resolve", specifier: specifier, baseURL: baseURL}, '*');
+    parsedImportMap.contentWindow.postMessage({action: 'prepareResolve'}, '*');
+
+    navigator.serviceWorker.addEventListener('message', event => {
+      // Step 2. After postMessage() at Step 1 is processed, the service worker
+      // sends back a message and the parent Window receives the message here
+      // and sends a 'resolve' message to the iframe.
+      parsedImportMap.contentWindow.postMessage(
+          {action: 'resolve',
+           specifier: specifier,
+           baseURL: baseURL,
+           expectedURL: expectedURL,
+           useInternalMethods: window.useInternalMethods},
+          '*');
+    }, {once: true});
   });
 }
 
@@ -112,14 +118,15 @@ function getParsedImportMap(parsedImportMap) {
       {once: true});
 
     parsedImportMap.contentWindow.postMessage(
-        {action: "getParsedImportMap"}, '*');
+        {action: 'getParsedImportMap',
+         useInternalMethods: window.useInternalMethods}, '*');
   });
 }
 
 function assert_no_extra_properties(object, expectedProperties, description) {
   for (const actualProperty in object) {
     assert_true(expectedProperties.indexOf(actualProperty) !== -1,
-        description + ": unexpected property " + actualProperty);
+        description + ': unexpected property ' + actualProperty);
   }
 }
 
@@ -184,18 +191,18 @@ async function runTests(j) {
       assert_own_property(j, 'baseURL');
       assert_equals(
           j.parsedImportMap.parseImportMapResult,
-          "Success",
-          "Import map registration should be successful for resolution tests");
+          'Success',
+          'Import map registration should be successful for resolution tests');
       for (const specifier in j.expectedResults) {
         const expected = j.expectedResults[specifier];
         promise_test(async t => {
             if (expected === null) {
               return promise_rejects_js(t, TypeError,
-                  resolve(specifier, j.parsedImportMap, j.baseURL));
+                  resolve(specifier, j.parsedImportMap, j.baseURL, null));
             } else {
               // Should be resolved to `expected`.
               const actual = await resolve(
-                  specifier, j.parsedImportMap, j.baseURL);
+                  specifier, j.parsedImportMap, j.baseURL, expected);
               assert_equals(actual, expected);
             }
           },
@@ -207,7 +214,7 @@ async function runTests(j) {
     if (j.hasOwnProperty('expectedParsedImportMap')) {
       promise_test(async t => {
         if (j.expectedParsedImportMap === null) {
-          assert_equals(j.parsedImportMap.parseImportMapResult, "ParseError");
+          assert_equals(j.parsedImportMap.parseImportMapResult, 'ParseError');
         } else {
           const actualParsedImportMap =
               await getParsedImportMap(j.parsedImportMap);
diff --git a/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map-internal.json b/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map-internal.json
new file mode 100644
index 00000000000..59390c89f6b
--- /dev/null
+++ b/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map-internal.json
@@ -0,0 +1,20 @@
+{
+  "importMap": {},
+  "importMapBaseURL": "https://example.com/app/index.html",
+  "baseURL": "https://example.com/js/app.mjs",
+  "tests": {
+    "non-HTTPS fetch scheme absolute URLs": {
+      "expectedResults": {
+        "about:fetch-scheme": "about:fetch-scheme"
+      }
+    },
+    "non-fetch scheme absolute URLs": {
+      "expectedResults": {
+        "mailto:non-fetch-scheme": "mailto:non-fetch-scheme",
+        "import:non-fetch-scheme": "import:non-fetch-scheme",
+        "javascript:non-fetch-scheme": "javascript:non-fetch-scheme",
+        "wss:non-fetch-scheme": "wss://non-fetch-scheme/"
+      }
+    }
+  }
+}
diff --git a/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map.json b/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map.json
index ce6c185498f..ff85a6d69d5 100644
--- a/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map.json
+++ b/tests/wpt/web-platform-tests/import-maps/common/resources/empty-import-map.json
@@ -18,23 +18,14 @@
         "/../foo/../bar": "https://example.com/bar"
       }
     },
-    "fetch scheme absolute URLs": {
+    "HTTPS scheme absolute URLs": {
       "expectedResults": {
-        "about:fetch-scheme": "about:fetch-scheme",
         "https://fetch-scheme.net": "https://fetch-scheme.net/",
         "https:fetch-scheme.org": "https://fetch-scheme.org/",
         "https://fetch%2Dscheme.com/": "https://fetch-scheme.com/",
         "https://///fetch-scheme.com///": "https://fetch-scheme.com///"
       }
     },
-    "non-fetch scheme absolute URLs": {
-      "expectedResults": {
-        "mailto:non-fetch-scheme": "mailto:non-fetch-scheme",
-        "import:non-fetch-scheme": "import:non-fetch-scheme",
-        "javascript:non-fetch-scheme": "javascript:non-fetch-scheme",
-        "wss:non-fetch-scheme": "wss://non-fetch-scheme/"
-      }
-    },
     "valid relative URLs that are invalid as specifiers should fail": {
       "expectedResults": {
         "invalid-specifier": null,
diff --git a/tests/wpt/web-platform-tests/lint.ignore b/tests/wpt/web-platform-tests/lint.ignore
index fe2933bb8a3..b44faa491f9 100644
--- a/tests/wpt/web-platform-tests/lint.ignore
+++ b/tests/wpt/web-platform-tests/lint.ignore
@@ -690,7 +690,7 @@ MISSING-LINK: css/geometry/*.worker.js
 MISSING-LINK: css/filter-effects/*.any.js
 
 # Tests that use WebKit/Blink testing APIs
-LAYOUTTESTS APIS: import-maps/common/resources/common-test-helper.js
+LAYOUTTESTS APIS: import-maps/common/resources/common-test-helper-iframe.js
 LAYOUTTESTS APIS: resources/chromium/enable-hyperlink-auditing.js
 LAYOUTTESTS APIS: resources/chromium/generic_sensor_mocks.js
 LAYOUTTESTS APIS: resources/chromium/webxr-test.js
diff --git a/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-play-terminate-worker.html b/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-play-terminate-worker.html
new file mode 100644
index 00000000000..d1c1c3d3fe8
--- /dev/null
+++ b/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-play-terminate-worker.html
@@ -0,0 +1,74 @@
+<!DOCTYPE html>
+<html>
+<title>MediaSource-in-Worker looped playback test case with worker termination at various places</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body>
+<script>
+
+function terminateWorkerAfterMultipleSetTimeouts(test, worker, timeouts_remaining) {
+  if (timeouts_remaining <= 0) {
+    worker.terminate();
+    test.step_timeout(() => { test.done(); }, 0);
+  } else {
+    test.step_timeout(() => {
+      terminateWorkerAfterMultipleSetTimeouts(test, worker, --timeouts_remaining);
+    }, 0);
+  }
+}
+
+function startWorkerAndTerminateWorker(test, when_to_start_timeouts, timeouts_to_await) {
+  const worker = new Worker("mediasource-worker-util.js");
+  worker.onerror = test.unreached_func("worker error");
+
+  const video = document.createElement("video");
+  document.body.appendChild(video);
+  video.onerror = test.unreached_func("video element error");
+
+  if (when_to_start_timeouts == "after first ended event") {
+    video.addEventListener("ended", test.step_func(() => {
+      terminateWorkerAfterMultipleSetTimeouts(test, worker, timeouts_to_await);
+      video.currentTime = 0;
+      video.loop = true;
+    }), { once : true });
+  } else {
+    video.loop = true;
+  }
+
+  if (when_to_start_timeouts == "before setting src") {
+    terminateWorkerAfterMultipleSetTimeouts(test, worker, timeouts_to_await);
+  }
+
+  worker.onmessage = test.step_func((e) => {
+    if (e.data.substr(0,6) == "Error:") {
+      assert_unreached("Worker error: " + e.data);
+    } else {
+      const url = e.data
+      assert_true(url.match(/^blob:.+/) != null);
+      video.src = url;
+      if (when_to_start_timeouts == "after setting src") {
+        terminateWorkerAfterMultipleSetTimeouts(test, worker, timeouts_to_await);
+      }
+      video.play().catch(error => {
+        // Only rejections due to MEDIA_ERR_SRC_NOT_SUPPORTED are expected to possibly
+        // occur, except if we expect to reach at least 1 'ended' event.
+        assert_not_equals(when_to_start_timeouts, "after first ended event");
+        assert_true(video.error != null);
+        assert_equals(video.error.code, MediaError.MEDIA_ERR_SRC_NOT_SUPPORTED);
+        // Do not rethrow. Instead, wait for the step_timeouts to finish the test.
+      });
+    }
+  });
+}
+
+[ "before setting src", "after setting src", "after first ended event" ].forEach((when) => {
+  for (let timeouts = 0; timeouts < 10; ++timeouts) {
+    async_test((test) => { startWorkerAndTerminateWorker(test, when, timeouts); },
+        "Test worker MediaSource termination after at least " + timeouts +
+          " main thread setTimeouts, starting counting " + when);
+  }
+});
+
+</script>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-attach.html b/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-play.html
similarity index 60%
rename from tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-attach.html
rename to tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-play.html
index b09f05c2485..200f8a851bf 100644
--- a/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-attach.html
+++ b/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-play.html
@@ -1,6 +1,6 @@
 <!DOCTYPE html>
 <html>
-<title>Test attachment to dedicated worker MediaSource</title>
+<title>Simple MediaSource-in-Worker playback test case</title>
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <body>
@@ -9,15 +9,8 @@
 async_test((t) => {
   const video = document.createElement('video');
   document.body.appendChild(video);
-
-  // TODO(https://crbug.com/878133): Enable attachment success by
-  // completing the CrossThreadAttachment implementation. Currently,
-  // a custom Chromium MediaError.message is confirmed.
-  video.onerror = t.step_func(() => {
-    assert_not_equals(video.error, null);
-    assert_equals(video.error.message, "MEDIA_ELEMENT_ERROR: Unable to attach MediaSource");
-    t.done();
-  });
+  video.onerror = t.unreached_func("video element error");
+  video.onended = t.step_func_done();
 
   let worker = new Worker("mediasource-worker-util.js");
   worker.onerror = t.unreached_func("worker error");
@@ -28,9 +21,10 @@ async_test((t) => {
       const url = e.data;
       assert_true(url.match(/^blob:.+/) != null);
       video.src = url;
+      video.play();
     }
   });
-}, "Test worker MediaSource attachment (currently should fail to attach)");
+}, "Test worker MediaSource construction, attachment, buffering and basic playback");
 
 // TODO(https://crbug.com/878133): Test multiple attachments to same worker
 // MediaSource racing each other: precisely one should win the race.
diff --git a/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-util.js b/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-util.js
index e0c4bdc0e10..4cee4862fa4 100644
--- a/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-util.js
+++ b/tests/wpt/web-platform-tests/media-source/dedicated-worker/mediasource-worker-util.js
@@ -13,11 +13,11 @@ let mediaLoad;
 // Find supported test media, if any.
 let MEDIA_LIST = [
   {
-    url: 'mp4/test.mp4',
+    url: '../mp4/test.mp4',
     type: 'video/mp4; codecs="mp4a.40.2,avc1.4d400d"',
   },
   {
-    url: 'webm/test.webm',
+    url: '../webm/test.webm',
     type: 'video/webm; codecs="vp8, vorbis"',
   },
 ];
@@ -56,9 +56,6 @@ onmessage = function(evt) {
   postMessage("Error: No message expected by Worker");
 };
 
-// TODO(https://crbug.com/878133): Enable this path by completing the
-// CrossThreadMediaSourceAttachment implementation such that attachment can
-// actually succeed and 'sourceopen' be dispatched.
 mediaSource.addEventListener("sourceopen", () => {
   URL.revokeObjectURL(mediaSourceObjectUrl);
   sourceBuffer = mediaSource.addSourceBuffer(mediaMetadata.type);
@@ -66,10 +63,16 @@ mediaSource.addEventListener("sourceopen", () => {
     postMessage("Error: " + err);
   };
   sourceBuffer.onupdateend = () => {
+    // Reset the parser. Unnecessary for this buffering, except helps with test
+    // coverage.
+    sourceBuffer.abort();
     // Shorten the buffered media and test playback duration to avoid timeouts.
     sourceBuffer.remove(0.5, Infinity);
     sourceBuffer.onupdateend = () => {
       sourceBuffer.duration = 0.5;
+      // Issue changeType to the same type that we've already buffered.
+      // Unnecessary for this buffering, except helps with test coverage.
+      sourceBuffer.changeType(mediaMetadata.type);
       mediaSource.endOfStream();
     };
   };
diff --git a/tests/wpt/web-platform-tests/scroll-animations/element-based-offset-clamp.html b/tests/wpt/web-platform-tests/scroll-animations/element-based-offset-clamp.html
index 6467af199f1..b1405074938 100644
--- a/tests/wpt/web-platform-tests/scroll-animations/element-based-offset-clamp.html
+++ b/tests/wpt/web-platform-tests/scroll-animations/element-based-offset-clamp.html
@@ -7,19 +7,25 @@
 <script src="testcommon.js"></script>
 
 <style>
-/*
- * Overflow hidden prevents user scroll including mouse wheel; however, the
- * element is still a scrollable container and can be scrolled programmatically.
- * Removing the visible scrollbars in this manner simplifies the position
- * calculations in the text.
- */
 .scroller {
-  overflow: hidden;
+  overflow: scroll;
   height: 500px;
   width: 500px;
   will-change: transform;
 }
 
+/* Disable scrollbars to simplify the calculations in the test. */
+.scroller {
+  scrollbar-width: 0;
+}
+/*
+Chrome does not support scrollbar-width so we use this non-standard property
+until it does.
+*/
+.scroller::-webkit-scrollbar {
+  display: none;
+}
+
 .contents {
   height: 1200px;
   width: 1200px;
@@ -206,4 +212,4 @@
       createScrollAnimationTest(description, config);
     }
   }
-</script>
+</script>
\ No newline at end of file