diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index 4cf4e5f5062..f1a958b3ff7 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -178119,6 +178119,42 @@
      {}
     ]
    ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-button-001.html": [
     [
      "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-button-001.html",
@@ -178191,6 +178227,42 @@
      {}
     ]
    ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-flex-001.html": [
     [
      "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-flex-001.html",
@@ -179895,6 +179967,18 @@
      {}
     ]
    ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005.html": [
+    [
+     "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005.html",
+     [
+      [
+       "/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-dyn-resize-001.html": [
     [
      "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-dyn-resize-001.html",
@@ -284969,6 +285053,21 @@
      {}
     ]
    ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002-ref.html": [
+    [
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003-ref.html": [
+    [
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004-ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-button-001-ref.html": [
     [
      {}
@@ -284999,6 +285098,21 @@
      {}
     ]
    ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002-ref.html": [
+    [
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003-ref.html": [
+    [
+     {}
+    ]
+   ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004-ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-flex-001-ref.html": [
     [
      {}
@@ -285749,6 +285863,11 @@
      {}
     ]
    ],
+   "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005-ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-dyn-resize-001-ref.html": [
     [
      {}
@@ -633033,7 +633152,7 @@
    "reftest"
   ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/background/reftest.list": [
-   "e60bdb947f53f92819bfbc507cf11e683ee82e92",
+   "cdaad122cbe1c9ded075a0239ec3d26d01a3d2e8",
    "support"
   ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/background/support/aqua-yellow-32x32.png": [
@@ -633572,6 +633691,30 @@
    "0ed82ff8fc5b8adaeef2bfd973cd79973b8f4fa6",
    "reftest"
   ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002-ref.html": [
+   "1f41028faac4dfb444745567e4ac9087fefbea37",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002.html": [
+   "4dc92103eabc6231b4d8260cbb35ac7337b2362b",
+   "reftest"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003-ref.html": [
+   "2f6ff8322d13942bfff3c54b6a3f132a05cdfb5a",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003.html": [
+   "cf7e2310169c57658ebc688db3a927152868d97a",
+   "reftest"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004-ref.html": [
+   "c8647b19dd6c5b64a0aab86ff2f394f93ac7ff3d",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004.html": [
+   "b6cca54a8c65ca798b836aa7b6ea92627482dc76",
+   "reftest"
+  ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-button-001-ref.html": [
    "efadf9b5df49ca45f829208c03dc0feb875c74f1",
    "support"
@@ -633620,6 +633763,30 @@
    "41458550272f896d47e15210c82f4c22d386d9e0",
    "reftest"
   ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002-ref.html": [
+   "3d0f38f7c07c4b73cc41418c87e5bc902e7ed862",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002.html": [
+   "d3a69e5f357bef37bcd2dbb3a0877ea4d8bdc977",
+   "reftest"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003-ref.html": [
+   "6079764591ff8806c88bc2644c4248e12ea0ca0a",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003.html": [
+   "7ea22635d35b4471362545d0abcf7f417511323d",
+   "reftest"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004-ref.html": [
+   "e7f03ff2d31e30b837b654ba41de220daef1c753",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004.html": [
+   "1de39348b718cd90530be9a0089b4075f3190a8f",
+   "reftest"
+  ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-flex-001-ref.html": [
    "93a263b5b2ffc131bdfe4989c3d90c312cadf6e4",
    "support"
@@ -633661,7 +633828,7 @@
    "reftest"
   ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/contain/reftest.list": [
-   "c99a7e5af12855a9473685d78145e3c7871c9223",
+   "eaf31a7dc12937847b0d850dadb3ad0ee6cadcc4",
    "support"
   ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/dependent-builtin-ref.html": [
@@ -634960,6 +635127,14 @@
    "cf54aabe9936b3963e7343d566d957633fc26c69",
    "reftest"
   ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005-ref.html": [
+   "fca146671cfb80cd3ae0264a8466be0b9823b3ae",
+   "support"
+  ],
+  "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005.html": [
+   "76beb2a41db70d2f00de159d1285fbc5310a3d40",
+   "reftest"
+  ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-dyn-resize-001-ref.html": [
    "5e5561cadf2ddbd6dfd2c7b4819f146f61be006b",
    "support"
@@ -635821,7 +635996,7 @@
    "reftest"
   ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/reftest.list": [
-   "850651a9f6ec839261e95261b5bd2fdffae6f4a8",
+   "6f90914621b37a9aa9c627f0b4cdbd9aeba679e2",
    "support"
   ],
   "css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/support/Ahem.ttf": [
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 18f786d4963..43c9d36c8fd 100644
--- a/tests/wpt/metadata/fetch/content-type/response.window.js.ini
+++ b/tests/wpt/metadata/fetch/content-type/response.window.js.ini
@@ -312,3 +312,6 @@
   [<iframe>: separate response Content-Type: text/plain */*;charset=gbk]
     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 87c807a49ff..536384f36e1 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%0C]
     expected: FAIL
 
+  [X-Content-Type-Options%3A%20%2Cnosniff]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/browsers/browsing-the-web/navigating-across-documents/javascript-url-abort/javascript-url-abort-return-value-string.tentative.html.ini b/tests/wpt/metadata/html/browsers/browsing-the-web/navigating-across-documents/javascript-url-abort/javascript-url-abort-return-value-string.tentative.html.ini
new file mode 100644
index 00000000000..9bacb9f5c4e
--- /dev/null
+++ b/tests/wpt/metadata/html/browsers/browsing-the-web/navigating-across-documents/javascript-url-abort/javascript-url-abort-return-value-string.tentative.html.ini
@@ -0,0 +1,4 @@
+[javascript-url-abort-return-value-string.tentative.html]
+  [Aborting fetch for javascript:string navigation]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_5.html.ini b/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_5.html.ini
new file mode 100644
index 00000000000..dc2e45516de
--- /dev/null
+++ b/tests/wpt/metadata/html/browsers/history/the-history-interface/traverse_the_history_5.html.ini
@@ -0,0 +1,4 @@
+[traverse_the_history_5.html]
+  [Multiple history traversals, last would be aborted]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/the-img-element/non-active-document.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/the-img-element/non-active-document.html.ini
deleted file mode 100644
index 8cc42056d34..00000000000
--- a/tests/wpt/metadata/html/semantics/embedded-content/the-img-element/non-active-document.html.ini
+++ /dev/null
@@ -1,10 +0,0 @@
-[non-active-document.html]
-  [DOMParser]
-    expected: FAIL
-
-  [createHTMLDocument]
-    expected: FAIL
-
-  [<template>]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/html/semantics/interactive-elements/the-details-element/toggleEvent.html.ini b/tests/wpt/metadata/html/semantics/interactive-elements/the-details-element/toggleEvent.html.ini
new file mode 100644
index 00000000000..9e522297c94
--- /dev/null
+++ b/tests/wpt/metadata/html/semantics/interactive-elements/the-details-element/toggleEvent.html.ini
@@ -0,0 +1,7 @@
+[toggleEvent.html]
+  [Calling open twice on 'details' fires only one toggle event]
+    expected: FAIL
+
+  [Setting open=true to opened 'details' element should not fire a toggle event at the 'details' element]
+    expected: FAIL
+
diff --git a/tests/wpt/mozilla/meta/mozilla/scroll_root.html.ini b/tests/wpt/mozilla/meta/mozilla/scroll_root.html.ini
deleted file mode 100644
index ef9d00edd6e..00000000000
--- a/tests/wpt/mozilla/meta/mozilla/scroll_root.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[scroll_root.html]
-  expected: FAIL
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/background/reftest.list b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/background/reftest.list
index e60bdb947f5..cdaad122cbe 100644
--- a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/background/reftest.list
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/background/reftest.list
@@ -38,4 +38,4 @@
 == background-attachment-fixed-inside-transform-1.html background-attachment-fixed-inside-transform-1-ref.html
 
 # box-shadow with currentcolor test cases
-== box-shadow-currentcolor.html box-shadow-currentcolor-ref.html
\ No newline at end of file
+== box-shadow-currentcolor.html box-shadow-currentcolor-ref.html
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002-ref.html
new file mode 100644
index 00000000000..1f41028faac
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002-ref.html
@@ -0,0 +1,88 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Reftest Reference</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <style>
+  .basic {
+    overflow: scroll;
+    position: relative;
+    border: 2px solid green;
+  }
+  .height-ref {
+    height: 60px;
+    background: lightblue;
+  }
+  .width-ref {
+    width: 60px;
+  }
+  .floatLBasic-ref {
+    float: left;
+  }
+  .floatLWidth-ref {
+    float: left;
+    width: 60px;
+  }
+  .flexBaselineCheck {
+    display: flex;
+    align-items: baseline;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+    position: absolute;
+  }
+  .zeroHeightContents {
+    color: transparent;
+    height: 0px;
+    width: 0px;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In the reference-case scenarios here, we use the same DOM as in
+       the testcase, and we simply use 'position: absolute' on the descendants
+       wherever the testcase has 'contain: size' on the container.  This
+       produces an accurate reference rendering, because out-of-flow content
+       doesn't contribute to the container's sizing, but does create scrollable
+       overflow (i.e. it produces scrollbars of the appropriate size for the
+       amount of overflow). -->
+  <div class="basic"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic floatLBasic-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic floatLWidth-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="flexBaselineCheck">
+    outside before
+    <div class="basic">
+      <!-- We use the out-of-flow "innerContents" to create the correct
+           amount of scrollable overflow to match the testcase, and we
+           use the smaller in-flow "zeroHeightContents" to provide a
+           baseline that we can use to verify the testcase's baseline
+           alignment position. -->
+      <div class="innerContents">inner</div>
+      <div class="zeroHeightContents">i</div>
+    </div>
+    outside after
+  </div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002.html
new file mode 100644
index 00000000000..4dc92103eab
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-002.html
@@ -0,0 +1,93 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: 'contain: size' on 'overflow:scroll' block elements should cause them to be sized as if they had no contents</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <link rel="help" href="https://drafts.csswg.org/css-contain/#containment-size">
+  <link rel="match" href="contain-size-block-002-ref.html">
+  <style>
+  .contain {
+    contain: size;
+    overflow: scroll;
+    border: 2px solid green;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+  }
+  .minHeight {
+    min-height: 60px;
+    background: lightblue;
+  }
+  .height {
+    height: 60px;
+    background: lightblue;
+  }
+  .maxWidth {
+    max-width: 60px;
+  }
+  .width {
+    width: 60px;
+  }
+  .floatLBasic {
+    float: left;
+  }
+  .floatLWidth {
+    float: left;
+    width: 60px;
+  }
+  .flexBaselineCheck {
+    display: flex;
+    align-items: baseline;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In all cases below, the expectation is that the size-contained
+       element should be sized as if it had no contents (while honoring
+       whatever sizing properties are provided). -->
+
+  <!--CSS Test: A size-contained scrollable block with no specified size.-->
+  <div class="contain"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified min-height -->
+  <div class="contain minHeight"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified height -->
+  <div class="contain height"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified max-width -->
+  <div class="contain maxWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified width -->
+  <div class="contain width"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained floated scrollable block with auto size -->
+  <div class="contain floatLBasic"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained floated scrollable block with
+      specified width -->
+  <div class="contain floatLWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block in a
+      baseline-aligning flex container: should size as if it's empty, but
+      still baseline-align using its contents' baseline -->
+  <div class="flexBaselineCheck">
+    outside before
+    <div class="contain">
+      <div class="innerContents">inner</div>
+    </div>
+    outside after
+  </div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003-ref.html
new file mode 100644
index 00000000000..2f6ff8322d1
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003-ref.html
@@ -0,0 +1,88 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Reftest Reference</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <style>
+  .basic {
+    overflow: auto;
+    position: relative;
+    border: 2px solid green;
+  }
+  .height-ref {
+    height: 60px;
+    background: lightblue;
+  }
+  .width-ref {
+    width: 60px;
+  }
+  .floatLBasic-ref {
+    float: left;
+  }
+  .floatLWidth-ref {
+    float: left;
+    width: 60px;
+  }
+  .flexBaselineCheck {
+    display: flex;
+    align-items: baseline;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+    position: absolute;
+  }
+  .zeroHeightContents {
+    color: transparent;
+    height: 0px;
+    width: 0px;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In the reference-case scenarios here, we use the same DOM as in
+       the testcase, and we simply use 'position: absolute' on the descendants
+       wherever the testcase has 'contain: size' on the container.  This
+       produces an accurate reference rendering, because out-of-flow content
+       doesn't contribute to the container's sizing, but does create scrollable
+       overflow (i.e. it produces scrollbars of the appropriate size for the
+       amount of overflow). -->
+  <div class="basic"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic floatLBasic-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic floatLWidth-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="flexBaselineCheck">
+    outside before
+    <div class="basic">
+      <!-- We use the out-of-flow "innerContents" to create the correct
+           amount of scrollable overflow to match the testcase, and we
+           use the smaller in-flow "zeroHeightContents" to provide a
+           baseline that we can use to verify the testcase's baseline
+           alignment position. -->
+      <div class="innerContents">inner</div>
+      <div class="zeroHeightContents">i</div>
+    </div>
+    outside after
+  </div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003.html
new file mode 100644
index 00000000000..cf7e2310169
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-003.html
@@ -0,0 +1,93 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: 'contain: size' on 'overflow:auto' block elements should cause them to be sized as if they had no contents</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <link rel="help" href="https://drafts.csswg.org/css-contain/#containment-size">
+  <link rel="match" href="contain-size-block-003-ref.html">
+  <style>
+  .contain {
+    contain: size;
+    overflow: auto;
+    border: 2px solid green;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+  }
+  .minHeight {
+    min-height: 60px;
+    background: lightblue;
+  }
+  .height {
+    height: 60px;
+    background: lightblue;
+  }
+  .maxWidth {
+    max-width: 60px;
+  }
+  .width {
+    width: 60px;
+  }
+  .floatLBasic {
+    float: left;
+  }
+  .floatLWidth {
+    float: left;
+    width: 60px;
+  }
+  .flexBaselineCheck {
+    display: flex;
+    align-items: baseline;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In all cases below, the expectation is that the size-contained
+       element should be sized as if it had no contents (while honoring
+       whatever sizing properties are provided). -->
+
+  <!--CSS Test: A size-contained scrollable block with no specified size.-->
+  <div class="contain"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified min-height -->
+  <div class="contain minHeight"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified height -->
+  <div class="contain height"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified max-width -->
+  <div class="contain maxWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified width -->
+  <div class="contain width"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained floated scrollable block with auto size -->
+  <div class="contain floatLBasic"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained floated scrollable block with
+      specified width -->
+  <div class="contain floatLWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block in a
+      baseline-aligning flex container: should size as if it's empty, but
+      still baseline-align using its contents' baseline -->
+  <div class="flexBaselineCheck">
+    outside before
+    <div class="contain">
+      <div class="innerContents">inner</div>
+    </div>
+    outside after
+  </div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004-ref.html
new file mode 100644
index 00000000000..c8647b19dd6
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004-ref.html
@@ -0,0 +1,87 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Reftest Reference</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <style>
+  .basic {
+    overflow: hidden;
+    position: relative;
+    border: 2px solid green;
+  }
+  .height-ref {
+    height: 60px;
+    background: lightblue;
+  }
+  .width-ref {
+    width: 60px;
+  }
+  .floatLBasic-ref {
+    float: left;
+  }
+  .floatLWidth-ref {
+    float: left;
+    width: 60px;
+  }
+  .flexBaselineCheck {
+    display: flex;
+    align-items: baseline;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+    position: absolute;
+  }
+  .zeroHeightContents {
+    color: transparent;
+    height: 0px;
+    width: 0px;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In the reference-case scenarios here, we use the same DOM as in
+       the testcase, and we simply use 'position: absolute' on the descendants
+       wherever the testcase has 'contain: size' on the container.  This
+       produces an accurate reference rendering, because out-of-flow content
+       doesn't contribute to the container's sizing, but does create scrollable
+       overflow. -->
+  <div class="basic"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic floatLBasic-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic floatLWidth-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="flexBaselineCheck">
+    outside before
+    <div class="basic">
+      <!-- We use the out-of-flow "innerContents" to create the correct
+           amount of scrollable overflow to match the testcase, and we
+           use the smaller in-flow "zeroHeightContents" to provide a
+           baseline that we can use to verify the testcase's baseline
+           alignment position. -->
+      <div class="innerContents">inner</div>
+      <div class="zeroHeightContents">i</div>
+    </div>
+    outside after
+  </div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004.html
new file mode 100644
index 00000000000..b6cca54a8c6
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-block-004.html
@@ -0,0 +1,93 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: 'contain: size' on 'overflow:hidden' block elements should cause them to be sized as if they had no contents</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <link rel="help" href="https://drafts.csswg.org/css-contain/#containment-size">
+  <link rel="match" href="contain-size-block-004-ref.html">
+  <style>
+  .contain {
+    contain: size;
+    overflow: hidden;
+    border: 2px solid green;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+  }
+  .minHeight {
+    min-height: 60px;
+    background: lightblue;
+  }
+  .height {
+    height: 60px;
+    background: lightblue;
+  }
+  .maxWidth {
+    max-width: 60px;
+  }
+  .width {
+    width: 60px;
+  }
+  .floatLBasic {
+    float: left;
+  }
+  .floatLWidth {
+    float: left;
+    width: 60px;
+  }
+  .flexBaselineCheck {
+    display: flex;
+    align-items: baseline;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In all cases below, the expectation is that the size-contained
+       element should be sized as if it had no contents (while honoring
+       whatever sizing properties are provided). -->
+
+  <!--CSS Test: A size-contained scrollable block with no specified size.-->
+  <div class="contain"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified min-height -->
+  <div class="contain minHeight"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified height -->
+  <div class="contain height"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified max-width -->
+  <div class="contain maxWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block with specified width -->
+  <div class="contain width"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained floated scrollable block with auto size -->
+  <div class="contain floatLBasic"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained floated scrollable block with
+      specified width -->
+  <div class="contain floatLWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!--CSS Test: A size-contained scrollable block in a
+      baseline-aligning flex container: should size as if it's empty, but
+      still baseline-align using its contents' baseline -->
+  <div class="flexBaselineCheck">
+    outside before
+    <div class="contain">
+      <div class="innerContents">inner</div>
+    </div>
+    outside after
+  </div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002-ref.html
new file mode 100644
index 00000000000..3d0f38f7c07
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002-ref.html
@@ -0,0 +1,56 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Reftest Reference</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <style>
+  .basic {
+    display: inline-block;
+    overflow: scroll;
+    position: relative;
+    border: 2px solid green;
+  }
+  .height-ref {
+    height: 60px;
+  }
+  .width-ref {
+    width: 60px;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+    position: absolute;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In the reference-case scenarios here, we use the same DOM as in
+       the testcase, and we simply use 'position: absolute' on the descendants
+       wherever the testcase has 'contain: size' on the container.  This
+       produces an accurate reference rendering, because out-of-flow content
+       doesn't contribute to the container's sizing, but does create scrollable
+       overflow (i.e. it produces scrollbars of the appropriate size for the
+       amount of overflow). -->
+  <div class="basic"><div class="innerContents">inner</div></div>
+  <br>
+
+  outside before
+  <div class="basic"><div class="innerContents">inner</div></div>
+  outside after
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002.html
new file mode 100644
index 00000000000..d3a69e5f357
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-002.html
@@ -0,0 +1,67 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: 'contain: size' on 'overflow:scroll' inline-block elements should cause them to be sized as if they had no contents and baseline-aligned regularly.</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <link rel="help" href="https://drafts.csswg.org/css-contain/#containment-size">
+  <link rel="match" href="contain-size-inline-block-002-ref.html">
+  <style>
+  .contain {
+    display: inline-block;
+    overflow: scroll;
+    contain:size;
+    border: 2px solid green;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+  }
+  .minHeight {
+    min-height: 60px;
+  }
+  .height {
+    height: 60px;
+  }
+  .minWidth {
+    min-width: 60px;
+  }
+  .width {
+    width: 60px;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In all cases below, the expectation is that the size-contained
+       element should be sized as if it had no contents (while honoring
+       whatever sizing properties are provided). -->
+
+  <!-- A size-contained scrollable inline-block with no specified size -->
+  <div class="contain"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block should perform baseline
+       alignment regularly, based on contents' baseline. -->
+  outside before
+  <div class="contain"><div class="innerContents">inner</div></div>
+  outside after
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified min-height -->
+  <div class="contain minHeight"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified height -->
+  <div class="contain height"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified min-width -->
+  <div class="contain minWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified width -->
+  <div class="contain width"><div class="innerContents">inner</div></div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003-ref.html
new file mode 100644
index 00000000000..6079764591f
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003-ref.html
@@ -0,0 +1,56 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Reftest Reference</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <style>
+  .basic {
+    display: inline-block;
+    overflow: auto;
+    position: relative;
+    border: 2px solid green;
+  }
+  .height-ref {
+    height: 60px;
+  }
+  .width-ref {
+    width: 60px;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+    position: absolute;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In the reference-case scenarios here, we use the same DOM as in
+       the testcase, and we simply use 'position: absolute' on the descendants
+       wherever the testcase has 'contain: size' on the container.  This
+       produces an accurate reference rendering, because out-of-flow content
+       doesn't contribute to the container's sizing, but does create scrollable
+       overflow (i.e. it produces scrollbars of the appropriate size for the
+       amount of overflow). -->
+  <div class="basic"><div class="innerContents">inner</div></div>
+  <br>
+
+  outside before
+  <div class="basic"><div class="innerContents">inner</div></div>
+  outside after
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003.html
new file mode 100644
index 00000000000..7ea22635d35
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-003.html
@@ -0,0 +1,67 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: 'contain: size' on 'overflow:auto' inline-block elements should cause them to be sized as if they had no contents and baseline-aligned regularly.</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <link rel="help" href="https://drafts.csswg.org/css-contain/#containment-size">
+  <link rel="match" href="contain-size-inline-block-003-ref.html">
+  <style>
+  .contain {
+    display: inline-block;
+    overflow: auto;
+    contain:size;
+    border: 2px solid green;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+  }
+  .minHeight {
+    min-height: 60px;
+  }
+  .height {
+    height: 60px;
+  }
+  .minWidth {
+    min-width: 60px;
+  }
+  .width {
+    width: 60px;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In all cases below, the expectation is that the size-contained
+       element should be sized as if it had no contents (while honoring
+       whatever sizing properties are provided). -->
+
+  <!-- A size-contained scrollable inline-block with no specified size -->
+  <div class="contain"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block should perform baseline
+       alignment regularly, based on contents' baseline. -->
+  outside before
+  <div class="contain"><div class="innerContents">inner</div></div>
+  outside after
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified min-height -->
+  <div class="contain minHeight"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified height -->
+  <div class="contain height"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified min-width -->
+  <div class="contain minWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified width -->
+  <div class="contain width"><div class="innerContents">inner</div></div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004-ref.html
new file mode 100644
index 00000000000..e7f03ff2d31
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004-ref.html
@@ -0,0 +1,55 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Reftest Reference</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <style>
+  .basic {
+    display: inline-block;
+    overflow: hidden;
+    position: relative;
+    border: 2px solid green;
+  }
+  .height-ref {
+    height: 60px;
+  }
+  .width-ref {
+    width: 60px;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+    position: absolute;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In the reference-case scenarios here, we use the same DOM as in
+       the testcase, and we simply use 'position: absolute' on the descendants
+       wherever the testcase has 'contain: size' on the container.  This
+       produces an accurate reference rendering, because out-of-flow content
+       doesn't contribute to the container's sizing, but does create scrollable
+       overflow. -->
+  <div class="basic"><div class="innerContents">inner</div></div>
+  <br>
+
+  outside before
+  <div class="basic"><div class="innerContents">inner</div></div>
+  outside after
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic height-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+  <br>
+
+  <div class="basic width-ref"><div class="innerContents">inner</div></div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004.html
new file mode 100644
index 00000000000..1de39348b71
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/contain-size-inline-block-004.html
@@ -0,0 +1,67 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: 'contain: size' on 'overflow:hidden' inline-block elements should cause them to be sized as if they had no contents and baseline-aligned regularly.</title>
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="author" title="Morgan Rae Reschenberg" href="mailto:mreschenberg@berkeley.edu">
+  <link rel="help" href="https://drafts.csswg.org/css-contain/#containment-size">
+  <link rel="match" href="contain-size-inline-block-004-ref.html">
+  <style>
+  .contain {
+    display: inline-block;
+    overflow: hidden;
+    contain:size;
+    border: 2px solid green;
+  }
+  .innerContents {
+    color: transparent;
+    height: 100px;
+    width: 100px;
+  }
+  .minHeight {
+    min-height: 60px;
+  }
+  .height {
+    height: 60px;
+  }
+  .minWidth {
+    min-width: 60px;
+  }
+  .width {
+    width: 60px;
+  }
+  </style>
+</head>
+<body>
+  <!-- NOTE: In all cases below, the expectation is that the size-contained
+       element should be sized as if it had no contents (while honoring
+       whatever sizing properties are provided). -->
+
+  <!-- A size-contained scrollable inline-block with no specified size -->
+  <div class="contain"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block should perform baseline
+       alignment regularly, based on contents' baseline. -->
+  outside before
+  <div class="contain"><div class="innerContents">inner</div></div>
+  outside after
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified min-height -->
+  <div class="contain minHeight"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified height -->
+  <div class="contain height"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified min-width -->
+  <div class="contain minWidth"><div class="innerContents">inner</div></div>
+  <br>
+
+  <!-- A size-contained scrollable inline-block with specified width -->
+  <div class="contain width"><div class="innerContents">inner</div></div>
+</body>
+</html>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/reftest.list b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/reftest.list
index c99a7e5af12..eaf31a7dc12 100644
--- a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/reftest.list
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/contain/reftest.list
@@ -19,7 +19,13 @@
 == contain-paint-stacking-context-001b.html contain-paint-stacking-context-001-ref.html
 == contain-size-button-001.html contain-size-button-001-ref.html
 == contain-size-block-001.html contain-size-block-001-ref.html
+== contain-size-block-002.html contain-size-block-002-ref.html
+== contain-size-block-003.html contain-size-block-003-ref.html
+== contain-size-block-004.html contain-size-block-004-ref.html
 == contain-size-inline-block-001.html contain-size-inline-block-001-ref.html
+== contain-size-inline-block-002.html contain-size-inline-block-002-ref.html
+== contain-size-inline-block-003.html contain-size-inline-block-003-ref.html
+== contain-size-inline-block-004.html contain-size-inline-block-004-ref.html
 == contain-size-flex-001.html contain-size-flex-001-ref.html
 == contain-size-inline-flex-001.html contain-size-inline-flex-001-ref.html
 == contain-size-grid-001.html contain-size-grid-001-ref.html
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005-ref.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005-ref.html
new file mode 100644
index 00000000000..fca146671cf
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005-ref.html
@@ -0,0 +1,20 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<link rel="author" href="mailto:dholbert@mozilla.com" title="Daniel Holbert">
+<style>
+  .outer {
+    margin-bottom: 2px;
+    height: 60px;
+  }
+  .inner {
+    background: lime;
+    height: 100%;
+  }
+</style>
+<p>Test passes if you see four 60px-tall lime rows (with platform-appropriate scrollbars on the last one).</p>
+
+<div class="outer"><div class="inner"></div></div>
+<div class="outer"><div class="inner"></div></div>
+<div class="outer"><div class="inner"></div></div>
+<div class="outer" style="overflow: scroll"><div class="inner"></div></div>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005.html b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005.html
new file mode 100644
index 00000000000..76beb2a41db
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-definite-sizes-005.html
@@ -0,0 +1,47 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: nested flex containers with definite max-height</title>
+<link rel="match" href="flexbox-definite-sizes-005-ref.html">
+<link rel="author" href="mailto:dholbert@mozilla.com" title="Daniel Holbert">
+<link rel="help" href="https://drafts.csswg.org/css-flexbox/#definite-sizes">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1503173">
+<style>
+  .horizFlex {
+    display: flex;
+    margin-bottom: 2px;
+  }
+  .vertFlex {
+    display: flex;
+    flex-grow: 1;
+    flex-direction: column;
+    min-height: 60px;
+  }
+  .item {
+    background: lime;
+    height: 100%;
+  }
+</style>
+<p>Test passes if you see four 60px-tall lime rows (with platform-appropriate scrollbars on the last one).</p>
+
+<div class="horizFlex">
+  <div class="vertFlex">
+    <div class="item"></div>
+  </div>
+</div>
+
+<div class="horizFlex">
+  <div class="vertFlex" style="overflow: hidden">
+    <div class="item"></div>
+  </div>
+</div>
+<div class="horizFlex">
+  <div class="vertFlex" style="overflow: auto">
+    <div class="item"></div>
+  </div>
+</div>
+
+<div class="horizFlex">
+  <div class="vertFlex" style="overflow: scroll">
+    <div class="item"></div>
+  </div>
+</div>
diff --git a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/reftest.list b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/reftest.list
index 850651a9f6e..6f90914621b 100644
--- a/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/reftest.list
+++ b/tests/wpt/web-platform-tests/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/reftest.list
@@ -154,6 +154,7 @@
 == flexbox-definite-sizes-002.html flexbox-definite-sizes-001-ref.html
 == flexbox-definite-sizes-003.html flexbox-definite-sizes-001-ref.html
 == flexbox-definite-sizes-004.html flexbox-definite-sizes-001-ref.html
+== flexbox-definite-sizes-005.html flexbox-definite-sizes-005-ref.html
 
 # Tests for flex items as (pseudo) stacking contexts
 == flexbox-items-as-stacking-contexts-001.xhtml flexbox-items-as-stacking-contexts-001-ref.xhtml