From 26a029d407be480d791972afb5975cf62c9360a6 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Fri, 19 Apr 2024 02:47:55 +0200
Subject: Adding upstream version 124.0.1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../tests/css/css-will-change/META.yml             |  4 ++
 .../green-square-100-by-100-offset-ref.html        | 12 ++++++
 .../green-square-100-by-100-ref.html               | 12 ++++++
 .../tests/css/css-will-change/inheritance.html     | 21 ++++++++++
 .../parsing/will-change-computed.html              | 29 +++++++++++++
 .../parsing/will-change-invalid.html               | 49 ++++++++++++++++++++++
 .../css-will-change/parsing/will-change-valid.html | 29 +++++++++++++
 .../will-change-abspos-cb-001-ref.html             | 24 +++++++++++
 .../css-will-change/will-change-abspos-cb-001.html | 30 +++++++++++++
 .../will-change-abspos-cb-002-ref.html             | 13 ++++++
 .../css-will-change/will-change-abspos-cb-002.html | 29 +++++++++++++
 .../will-change-abspos-cb-003-ref.html             | 13 ++++++
 .../css-will-change/will-change-abspos-cb-003.html | 29 +++++++++++++
 .../will-change-abspos-cb-dynamic-001.html         | 34 +++++++++++++++
 .../will-change-contents-crash.html                | 11 +++++
 .../will-change-fixedpos-cb-001-ref.html           | 13 ++++++
 .../will-change-fixedpos-cb-001.html               | 29 +++++++++++++
 .../will-change-fixedpos-cb-002-ref.html           | 12 ++++++
 .../will-change-fixedpos-cb-002.html               | 27 ++++++++++++
 .../will-change-fixedpos-cb-003-ref.html           | 31 ++++++++++++++
 .../will-change-fixedpos-cb-003.html               | 41 ++++++++++++++++++
 .../will-change-fixedpos-cb-004-ref.html           | 13 ++++++
 .../will-change-fixedpos-cb-004.html               | 29 +++++++++++++
 .../will-change-fixedpos-cb-005-ref.html           | 12 ++++++
 .../will-change-fixedpos-cb-005.html               | 27 ++++++++++++
 .../will-change-fixedpos-cb-006-ref.html           | 31 ++++++++++++++
 .../will-change-fixedpos-cb-006.html               | 41 ++++++++++++++++++
 .../will-change-fixpos-cb-contain-1.html           | 25 +++++++++++
 .../will-change-fixpos-cb-filter-1.html            | 25 +++++++++++
 .../will-change-fixpos-cb-height-1.html            | 24 +++++++++++
 .../will-change-fixpos-cb-offset-path-1.html       | 24 +++++++++++
 .../will-change-fixpos-cb-perspective-1.html       | 25 +++++++++++
 .../will-change-fixpos-cb-position-1.html          | 24 +++++++++++
 .../will-change-fixpos-cb-transform-1.html         | 25 +++++++++++
 .../will-change-fixpos-cb-transform-style-1.html   | 25 +++++++++++
 .../will-change-fixpos-cb-translate-1.html         | 24 +++++++++++
 ...will-change-fixpos-cb-webkit-perspective-1.html | 27 ++++++++++++
 .../will-change-inherit-dynamic.html               | 21 ++++++++++
 ...-change-stacking-context-backdrop-filter-1.html | 37 ++++++++++++++++
 .../will-change-stacking-context-clip-path-1.html  | 21 ++++++++++
 .../will-change-stacking-context-filter-1.html     | 21 ++++++++++
 .../will-change-stacking-context-height-1.html     | 20 +++++++++
 .../will-change-stacking-context-isolation-1.html  | 21 ++++++++++
 .../will-change-stacking-context-mask-1.html       | 21 ++++++++++
 ...l-change-stacking-context-mix-blend-mode-1.html | 21 ++++++++++
 ...will-change-stacking-context-offset-path-1.html | 20 +++++++++
 .../will-change-stacking-context-opacity-1.html    | 21 ++++++++++
 .../will-change-stacking-context-opacity-2.html    | 37 ++++++++++++++++
 ...will-change-stacking-context-perspective-1.html | 21 ++++++++++
 .../will-change-stacking-context-position-1.html   | 21 ++++++++++
 .../will-change-stacking-context-transform-1.html  | 21 ++++++++++
 ...-change-stacking-context-transform-style-1.html | 21 ++++++++++
 .../will-change-stacking-context-translate-1.html  | 20 +++++++++
 .../will-change-stacking-context-z-index-1.html    | 21 ++++++++++
 .../will-change-transform-add-content-ref.html     |  4 ++
 .../will-change-transform-add-content.html         | 13 ++++++
 ...-change-transform-huge-offset-scrolled-ref.html |  3 ++
 ...will-change-transform-huge-offset-scrolled.html | 19 +++++++++
 .../will-change-transform-image-ref.html           |  4 ++
 .../will-change-transform-image.html               |  4 ++
 .../will-change-transform-inline-ref.html          | 18 ++++++++
 .../will-change-transform-inline.html              | 26 ++++++++++++
 ...sform-zero-size-child-overflow-visible-ref.html |  2 +
 ...transform-zero-size-child-overflow-visible.html |  6 +++
 .../css-will-change/will-change-will-change-1.html | 17 ++++++++
 65 files changed, 1395 insertions(+)
 create mode 100644 testing/web-platform/tests/css/css-will-change/META.yml
 create mode 100644 testing/web-platform/tests/css/css-will-change/green-square-100-by-100-offset-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/green-square-100-by-100-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/inheritance.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/parsing/will-change-computed.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/parsing/will-change-invalid.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/parsing/will-change-valid.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-dynamic-001.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-contents-crash.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-contain-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-filter-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-height-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-offset-path-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-perspective-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-position-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-style-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-translate-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-webkit-perspective-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-inherit-dynamic.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-backdrop-filter-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-clip-path-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-filter-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-height-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-isolation-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mask-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mix-blend-mode-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-offset-path-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-2.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-perspective-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-position-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-style-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-translate-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-stacking-context-z-index-1.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-add-content-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-add-content.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-image-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-image.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-inline-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-inline.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible-ref.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible.html
 create mode 100644 testing/web-platform/tests/css/css-will-change/will-change-will-change-1.html

(limited to 'testing/web-platform/tests/css/css-will-change')

diff --git a/testing/web-platform/tests/css/css-will-change/META.yml b/testing/web-platform/tests/css/css-will-change/META.yml
new file mode 100644
index 0000000000..8d265f2498
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/META.yml
@@ -0,0 +1,4 @@
+spec: https://drafts.csswg.org/css-will-change/
+suggested_reviewers:
+  - dbaron
+  - tabatkins
diff --git a/testing/web-platform/tests/css/css-will-change/green-square-100-by-100-offset-ref.html b/testing/web-platform/tests/css/css-will-change/green-square-100-by-100-offset-ref.html
new file mode 100644
index 0000000000..fee7030b6d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/green-square-100-by-100-offset-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change reference</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px; margin: 100px 0 0 100px; background: green }
+</style>
+<body>
+  <div></div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/green-square-100-by-100-ref.html b/testing/web-platform/tests/css/css-will-change/green-square-100-by-100-ref.html
new file mode 100644
index 0000000000..c49bf5a21d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/green-square-100-by-100-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change reference</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px; background: green }
+</style>
+<body>
+  <div></div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/inheritance.html b/testing/web-platform/tests/css/css-will-change/inheritance.html
new file mode 100644
index 0000000000..6b4ed2aa54
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/inheritance.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>Inheritance of CSS will-change property</title>
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#property-index">
+<meta name="assert" content="will-change does not inherit.">
+<meta name="assert" content="will-change has initial value auto.">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/inheritance-testcommon.js"></script>
+</head>
+<body>
+<div id="container">
+<div id="target"></div>
+</div>
+<script>
+assert_not_inherited('will-change', 'auto', 'transform');
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/css-will-change/parsing/will-change-computed.html b/testing/web-platform/tests/css/css-will-change/parsing/will-change-computed.html
new file mode 100644
index 0000000000..667f084eab
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/parsing/will-change-computed.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Will Change Test: getComputedStyle().willChange</title>
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#propdef-will-change">
+<meta name="assert" content="will-change computed value is as specified.">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+</head>
+<body>
+<div id="target"></div>
+<script>
+test_computed_value("will-change", "auto");
+
+// <animateable-feature> = scroll-position | contents | <custom-ident>
+test_computed_value("will-change", "scroll-position");
+test_computed_value("will-change", "contents");
+test_computed_value("will-change", "transform");
+test_computed_value("will-change", "background-color");
+
+test_computed_value("will-change", "scroll-position, contents");
+test_computed_value("will-change", "scroll-position, transform");
+test_computed_value("will-change", "contents, transform");
+test_computed_value("will-change", "transform, background-color");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/css-will-change/parsing/will-change-invalid.html b/testing/web-platform/tests/css/css-will-change/parsing/will-change-invalid.html
new file mode 100644
index 0000000000..e8b0b01973
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/parsing/will-change-invalid.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Will Change Test: parsing will-change with invalid values</title>
+<link rel="author" title="Eric Willigers" href="mailto:ericwilligers@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#propdef-will-change">
+<meta name="assert" content="will-change only supports the grammar 'auto | <animateable-feature>#'.">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+</head>
+<body>
+<script>
+test_invalid_value("will-change", "auto transform");
+test_invalid_value("will-change", "auto, transform");
+test_invalid_value("will-change", "contents auto");
+test_invalid_value("will-change", "contents, auto");
+
+let excludedKeywords = [
+  // CSS-wide keywords are excluded from <custom-ident>
+  // https://drafts.csswg.org/css-values-4/#identifier-value
+  "initial",
+  "inherit",
+  "unset",
+  "revert",
+  "revert-layer",
+  "default",
+
+  // will-change additionally excludes the following from <custom-ident>
+  "will-change",
+  "none",
+  "all",
+];
+
+for (let keyword of excludedKeywords) {
+  test_invalid_value("will-change", `transform, ${keyword}`);
+  test_invalid_value("will-change", `${keyword}, transform`);
+  for (let k of excludedKeywords) {
+    test_invalid_value("will-change", `${keyword}, ${k}`);
+  }
+}
+
+test_invalid_value("will-change", "will-change");
+test_invalid_value("will-change", "none");
+test_invalid_value("will-change", "all");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/css-will-change/parsing/will-change-valid.html b/testing/web-platform/tests/css/css-will-change/parsing/will-change-valid.html
new file mode 100644
index 0000000000..b8a086b3e3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/parsing/will-change-valid.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Will Change Test: parsing will-change with valid values</title>
+<link rel="author" title="Eric Willigers" href="mailto:ericwilligers@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#propdef-will-change">
+<meta name="assert" content="will-change supports the full grammar 'auto | <animateable-feature>#'.">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+</head>
+<body>
+<script>
+test_valid_value("will-change", "auto");
+
+// <animateable-feature> = scroll-position | contents | <custom-ident>
+test_valid_value("will-change", "scroll-position");
+test_valid_value("will-change", "contents");
+test_valid_value("will-change", "transform");
+test_valid_value("will-change", "background-color");
+
+test_valid_value("will-change", "scroll-position, contents");
+test_valid_value("will-change", "scroll-position, transform");
+test_valid_value("will-change", "contents, transform");
+test_valid_value("will-change", "transform, background-color");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001-ref.html
new file mode 100644
index 0000000000..f81b9d2a29
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001-ref.html
@@ -0,0 +1,24 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  .container {
+    border: 1px solid green;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+    display: flex;
+    position: relative;
+  }
+  .abspos {
+    position: absolute;
+    top: 0;
+    left: 0;
+    background: orange;
+    height: 20px;
+    width: 20px;
+  }
+</style>
+<div class="container">
+  <div class="abspos"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001.html
new file mode 100644
index 0000000000..d59e443310
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-001.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: position turns an element in an abspos containing block.</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Boris Zbarsky" href="mailto:bzbarsky@mit.edu">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1498873">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="will-change-abspos-cb-001-ref.html">
+<style>
+  .container {
+    border: 1px solid green;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+    display: flex;
+    will-change: position;
+  }
+  .abspos {
+    position: absolute;
+    top: 0;
+    left: 0;
+    background: orange;
+    height: 20px;
+    width: 20px;
+  }
+</style>
+<div class="container">
+  <div class="abspos"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002-ref.html
new file mode 100644
index 0000000000..9dfc24c526
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002-ref.html
@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  div {
+    border: 1px solid green;
+    background: green;
+    margin-top: 100px;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div></div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002.html
new file mode 100644
index 0000000000..537f671cdc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-002.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: filter should generate a containing block for absolute positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects/#FilterProperty">
+<link rel="match" href="will-change-abspos-cb-002-ref.html">
+<style>
+  .container {
+    border: 1px solid green;
+    background: red;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+    will-change: filter;
+  }
+  .abspos {
+    position: absolute;
+    top: 0;
+    left: 0;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<div class="container">
+  <div class="abspos"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003-ref.html
new file mode 100644
index 0000000000..9dfc24c526
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003-ref.html
@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  div {
+    border: 1px solid green;
+    background: green;
+    margin-top: 100px;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div></div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003.html
new file mode 100644
index 0000000000..d5da92db4f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-003.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: backdrop-filter should generate a containing block for absolute positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects-2/#BackdropFilterProperty">
+<link rel="match" href="will-change-abspos-cb-003-ref.html">
+<style>
+  .container {
+    border: 1px solid green;
+    background: red;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+    will-change: backdrop-filter;
+  }
+  .abspos {
+    position: absolute;
+    top: 0;
+    left: 0;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<div class="container">
+  <div class="abspos"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-dynamic-001.html b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-dynamic-001.html
new file mode 100644
index 0000000000..d2503256bc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-abspos-cb-dynamic-001.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: position turns an element in an abspos containing block when changed dynamically.</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1499863">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="will-change-abspos-cb-001-ref.html">
+<style>
+  .container {
+    border: 1px solid green;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+  }
+  .abspos {
+    position: absolute;
+    top: 0;
+    left: 0;
+    background: orange;
+    height: 20px;
+    width: 20px;
+  }
+</style>
+<div class="container">
+  <div class="abspos"></div>
+</div>
+<script>
+onload = function() {
+  let container = document.querySelector(".container");
+  container.offsetTop;
+  container.style.willChange = "position";
+}
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-contents-crash.html b/testing/web-platform/tests/css/css-will-change/will-change-contents-crash.html
new file mode 100644
index 0000000000..b2a05dac86
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-contents-crash.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<head>
+  <title>CSS will-change contents: Chrome crash with partial cache hit</title>
+  <link rel="author" href="mailto:sesse@chromium.org">
+  <link rel="help" href="https://crbug.com/1469858">
+</head>
+<body>
+  <div style="color: blue;"><div style="will-change: contents;">Test passes</div></div>
+  <div style="color: fuchsia;"><div style="will-change: contents;">if it does not crash.</div></div>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001-ref.html
new file mode 100644
index 0000000000..9dfc24c526
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001-ref.html
@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  div {
+    border: 1px solid green;
+    background: green;
+    margin-top: 100px;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div></div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001.html
new file mode 100644
index 0000000000..249413b6d7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-001.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: filter should generate a containing block for fixed positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects/#FilterProperty">
+<link rel="match" href="will-change-fixedpos-cb-001-ref.html">
+<style>
+  .container {
+    border: 1px solid green;
+    background: red;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+    will-change: filter;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 0;
+    left: 0;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<div class="container">
+  <div class="fixedpos"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002-ref.html
new file mode 100644
index 0000000000..7823c16075
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002-ref.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  div {
+    background: green;
+    margin-left: 100px;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div></div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002.html
new file mode 100644
index 0000000000..00cba5ecf8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-002.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: filter on an inline should generate a containing block for fixed positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects/#FilterProperty">
+<link rel="match" href="will-change-fixedpos-cb-002-ref.html">
+<style>
+  .inline {
+    margin-left: 100px;
+    will-change: filter;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 0;
+    left: 0;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<span class="inline">
+  <!-- "FAIL" should be hidden by the green box and should not be visible. -->
+  FAIL
+  <div class="fixedpos"></div>
+</span>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003-ref.html
new file mode 100644
index 0000000000..05957327c3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003-ref.html
@@ -0,0 +1,31 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  body {
+    margin: 0;
+  }
+  .hidden {
+    width: 100px;
+    height: 100px;
+    margin-left: 100px;
+    margin-top: 200px;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 100px;
+    left: 100px;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+  .spacer {
+    height: 2000px;
+  }
+</style>
+<div class="hidden"></div>
+<div class="fixedpos"></div>
+<div class="spacer"></div>
+<script>
+  window.onload = function() { window.scrollTo(0, 100); };
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003.html
new file mode 100644
index 0000000000..e7dcf2c880
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-003.html
@@ -0,0 +1,41 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: filter on the root should not generate a containing block for fixed positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects/#FilterProperty">
+<link rel="match" href="will-change-fixedpos-cb-003-ref.html">
+<style>
+  html {
+    will-change: filter;
+  }
+  body {
+    margin: 0;
+  }
+  .hidden {
+    width: 100px;
+    height: 100px;
+    background: red;
+    margin-left: 100px;
+    margin-top: 200px;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 100px;
+    left: 100px;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+  .spacer {
+    height: 2000px;
+  }
+</style>
+<!-- This object should be fully hidden by the fixed position object. -->
+<div class="hidden"></div>
+<div class="fixedpos"></div>
+<div class="spacer"></div>
+<script>
+  window.onload = function() { window.scrollTo(0, 100); };
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004-ref.html
new file mode 100644
index 0000000000..9dfc24c526
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004-ref.html
@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  div {
+    border: 1px solid green;
+    background: green;
+    margin-top: 100px;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div></div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004.html
new file mode 100644
index 0000000000..7bc5d92336
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-004.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: backdrop-filter should generate a containing block for fixed positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects-2/#BackdropFilterProperty">
+<link rel="match" href="will-change-fixedpos-cb-004-ref.html">
+<style>
+  .container {
+    border: 1px solid green;
+    background: red;
+    width: 100px;
+    height: 100px;
+    margin-top: 100px;
+    will-change: backdrop-filter;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 0;
+    left: 0;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<div class="container">
+  <div class="fixedpos"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005-ref.html
new file mode 100644
index 0000000000..7823c16075
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005-ref.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  div {
+    background: green;
+    margin-left: 100px;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div></div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005.html
new file mode 100644
index 0000000000..7cd88bd377
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-005.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: backdrop-filter on an inline should generate a containing block for fixed positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects-2/#BackdropFilterProperty">
+<link rel="match" href="will-change-fixedpos-cb-005-ref.html">
+<style>
+  .inline {
+    margin-left: 100px;
+    will-change: backdrop-filter;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 0;
+    left: 0;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<span class="inline">
+  <!-- "FAIL" should be hidden by the green box and should not be visible. -->
+  FAIL
+  <div class="fixedpos"></div>
+</span>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006-ref.html
new file mode 100644
index 0000000000..05957327c3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006-ref.html
@@ -0,0 +1,31 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<style>
+  body {
+    margin: 0;
+  }
+  .hidden {
+    width: 100px;
+    height: 100px;
+    margin-left: 100px;
+    margin-top: 200px;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 100px;
+    left: 100px;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+  .spacer {
+    height: 2000px;
+  }
+</style>
+<div class="hidden"></div>
+<div class="fixedpos"></div>
+<div class="spacer"></div>
+<script>
+  window.onload = function() { window.scrollTo(0, 100); };
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006.html b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006.html
new file mode 100644
index 0000000000..ec9eaa409f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixedpos-cb-006.html
@@ -0,0 +1,41 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: backdrop-filter on the root should not generate a containing block for fixed positioned elements.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects-2/#BackdropFilterProperty">
+<link rel="match" href="will-change-fixedpos-cb-006-ref.html">
+<style>
+  html {
+    will-change: backdrop-filter;
+  }
+  body {
+    margin: 0;
+  }
+  .hidden {
+    width: 100px;
+    height: 100px;
+    background: red;
+    margin-left: 100px;
+    margin-top: 200px;
+  }
+  .fixedpos {
+    position: fixed;
+    top: 100px;
+    left: 100px;
+    background: green;
+    height: 100px;
+    width: 100px;
+  }
+  .spacer {
+    height: 2000px;
+  }
+</style>
+<!-- This object should be fully hidden by the fixed position object. -->
+<div class="hidden"></div>
+<div class="fixedpos"></div>
+<div class="spacer"></div>
+<script>
+  window.onload = function() { window.scrollTo(0, 100); };
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-contain-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-contain-1.html
new file mode 100644
index 0000000000..3bb3afee3b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-contain-1.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: contain' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="https://drafts.csswg.org/css-containment/#containment-paint">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: contain; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-filter-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-filter-1.html
new file mode 100644
index 0000000000..afb03e8bec
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-filter-1.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: filter' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/filter-effects/#FilterProperty">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: filter; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-height-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-height-1.html
new file mode 100644
index 0000000000..892e6d4541
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-height-1.html
@@ -0,0 +1,24 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: height' does not create a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: height; margin: 100px 0 0 100px; background: red }
+.child { top: 100px; left: 100px; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 150px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-offset-path-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-offset-path-1.html
new file mode 100644
index 0000000000..0fe1ec1da7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-offset-path-1.html
@@ -0,0 +1,24 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: offset-path' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/motion-1/#offset-path-property">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: offset-path; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-perspective-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-perspective-1.html
new file mode 100644
index 0000000000..4a394f9958
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-perspective-1.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: perspective' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#perspective-property">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: perspective; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-position-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-position-1.html
new file mode 100644
index 0000000000..7e52ab4b03
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-position-1.html
@@ -0,0 +1,24 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: position' does not create a containing block for fixed positioned elements but does create a containing block for absolutely positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: position; margin: 100px 0 0 100px; background: red }
+.child { width: 50px; background: green }
+#fixpos { position: fixed; top: 100px; left: 100px }
+#abspos { position: absolute; top: 0; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-1.html
new file mode 100644
index 0000000000..92632cd62d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-1.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: transform' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#transform-property">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: transform; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-style-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-style-1.html
new file mode 100644
index 0000000000..808f7d8cd0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-transform-style-1.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: transform-style' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#transform-style-property">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: transform-style; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-translate-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-translate-1.html
new file mode 100644
index 0000000000..7ff0dfe768
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-translate-1.html
@@ -0,0 +1,24 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: translate' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="https://drafts.csswg.org/css-transforms-2/#propdef-translate">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: translate; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-webkit-perspective-1.html b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-webkit-perspective-1.html
new file mode 100644
index 0000000000..2438dcd975
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-fixpos-cb-webkit-perspective-1.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: -webkit-perspective' creates a containing block for fixed positioned elements</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="author" title="Google" href="https://www.google.com/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#perspective-property">
+<link rel="help" href="https://compat.spec.whatwg.org/#css-simple-aliases">
+<link rel="match" href="green-square-100-by-100-offset-ref.html">
+<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: -webkit-perspective; margin: 100px 0 0 100px; background: red }
+.child { top: 0; left: 0; width: 50px; background: green }
+#fixpos { position: fixed }
+#abspos { position: absolute; left: 50px }
+</style>
+<body>
+  <div id="wc">
+    <div class="child" id="fixpos">
+    </div>
+    <div class="child" id="abspos">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-inherit-dynamic.html b/testing/web-platform/tests/css/css-will-change/will-change-inherit-dynamic.html
new file mode 100644
index 0000000000..b01844aea5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-inherit-dynamic.html
@@ -0,0 +1,21 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: style changes are properly propagated to children if needed</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1499991">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="outer">
+  <div id="inner" style="will-change: inherit"></div>
+</div>
+<script>
+  test(function() {
+    assert_equals(getComputedStyle(outer).willChange, "auto");
+    assert_equals(getComputedStyle(inner).willChange, "auto");
+    outer.style.willChange = "color";
+    assert_equals(getComputedStyle(outer).willChange, "color");
+    assert_equals(getComputedStyle(inner).willChange, "color");
+  }, "will-change change is properly propagated to children if needed");
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-backdrop-filter-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-backdrop-filter-1.html
new file mode 100644
index 0000000000..0081e93c4f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-backdrop-filter-1.html
@@ -0,0 +1,37 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: backdrop-filter should create a stacking context.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=960953">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/filter-effects-2/#BackdropFilterProperty">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<style>
+  html, body { margin: 0; }
+  .indicator {
+    position: absolute;
+    background-color: green;
+    z-index: 1;
+  }
+  .willchange {
+    will-change: backdrop-filter;
+    z-index: 0;
+  }
+  .child {
+    position: relative;
+    background-color: red;
+    z-index: 2;
+  }
+  .box {
+    width: 100px;
+    height: 100px;
+  }
+</style>
+
+<div class="indicator box"></div>
+<div class="willchange box">
+  <!-- Because will-change: backdrop-filter creates a stacking context, this
+       child remains on bottom even though it has a higher z-index than the
+       indicator box. -->
+  <div class="child box"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-clip-path-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-clip-path-1.html
new file mode 100644
index 0000000000..673d1a895b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-clip-path-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: clip-path' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css-masking/#the-clip-path">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: clip-path; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-filter-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-filter-1.html
new file mode 100644
index 0000000000..0f87768db1
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-filter-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: filter' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/filter-effects/#FilterProperty">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: filter; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-height-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-height-1.html
new file mode 100644
index 0000000000..3ce0e61588
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-height-1.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: height' does not create a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: height; background: green }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: red }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-isolation-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-isolation-1.html
new file mode 100644
index 0000000000..9eaf812e93
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-isolation-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: isolation' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/compositing-1/#isolation">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: isolation; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mask-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mask-1.html
new file mode 100644
index 0000000000..77ec10caa9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mask-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: mask' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css-masking/#the-mask-image">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: mask; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mix-blend-mode-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mix-blend-mode-1.html
new file mode 100644
index 0000000000..ae13ac6b66
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-mix-blend-mode-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: mix-blend-mode' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/compositing-1/#mix-blend-mode">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: mix-blend-mode; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-offset-path-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-offset-path-1.html
new file mode 100644
index 0000000000..07277b66d9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-offset-path-1.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: offset-path' creates a stacking context</title>
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="https://drafts.fxtf.org/motion-1/#offset-path-property">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: offset-path; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-1.html
new file mode 100644
index 0000000000..2bff6676a7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: opacity' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="https://drafts.csswg.org/css-color-3/#transparency">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: opacity; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-2.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-2.html
new file mode 100644
index 0000000000..dfedbd2307
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-opacity-2.html
@@ -0,0 +1,37 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: will-change: opacity should create a stacking context for inlines.</title>
+<link rel="author" title="Philip Rogers" href="mailto:pdr@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1119651">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<style>
+  html, body { margin: 0; }
+  .indicator {
+    position: absolute;
+    width: 100px;
+    height: 100px;
+    background: green;
+    z-index: 1;
+  }
+  .will-change-opacity {
+    will-change: opacity;
+    width: 100px;
+    height: 100px;
+  }
+  .top {
+    position: absolute;
+    width: 100px;
+    height: 100px;
+    background: red;
+    z-index: 3;
+  }
+</style>
+
+<div class="indicator"></div>
+<span class="will-change-opacity">
+  <!-- Because will-change: opacity creates a stacking context, this child
+       remains on bottom even though it has a higher z-index than the indicator
+       box. -->
+  <div class="top"></div>
+</span>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-perspective-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-perspective-1.html
new file mode 100644
index 0000000000..3a6863220f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-perspective-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: perspective' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#perspective-property">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: perspective; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-position-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-position-1.html
new file mode 100644
index 0000000000..a244b7c2c5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-position-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: position' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-positioning/#sticky-pos">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: position; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-1.html
new file mode 100644
index 0000000000..1b55d1f7aa
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: transform' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#transform-property">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: transform; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-style-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-style-1.html
new file mode 100644
index 0000000000..f8e50c60c1
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-transform-style-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: transform-style' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-transforms/#transform-style-property">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: transform-style; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-translate-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-translate-1.html
new file mode 100644
index 0000000000..1e1e8355d6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-translate-1.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: translate' creates a stacking context</title>
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="https://drafts.csswg.org/css-transforms-2/#propdef-translate">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: translate; background: red }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-z-index-1.html b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-z-index-1.html
new file mode 100644
index 0000000000..b80338032c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-stacking-context-z-index-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS will-change: 'will-change: z-index' creates a stacking context</title>
+<link rel="author" title="L. David Baron" href="https://dbaron.org/">
+<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="help" href="http://www.w3.org/TR/css3-positioning/#layered-presentation">
+<link rel="match" href="green-square-100-by-100-ref.html">
+<meta name="assert" content="If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.">
+<style>
+html, body { margin: 0; padding: 0; }
+div { width: 100px; height: 100px }
+#wc { will-change: z-index; background: red; position: relative }
+#child { position: absolute; top: 0; left: 0; z-index: -1; background: green }
+</style>
+<body>
+  <div id="wc">
+    <div id="child">
+    </div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-add-content-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-add-content-ref.html
new file mode 100644
index 0000000000..edbeaa7b69
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-add-content-ref.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<div id="target" style="width: 200px; height: 100px; will-change: transform">
+This text should be visible.
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-add-content.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-add-content.html
new file mode 100644
index 0000000000..1d8568ee62
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-add-content.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="will-change-transform-add-content-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/rendering-utils.js"></script>
+<div id="target" style="width: 200px; height: 100px; will-change: transform"></div>
+<script>
+waitForAtLeastOneFrame().then(() => {
+  target.textContent = 'This text should be visible.';
+  takeScreenshot();
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled-ref.html
new file mode 100644
index 0000000000..c2af258e11
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled-ref.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<div style="width: 20px; height: 20px; margin-top: 10000px; background: green"></div>
+<script>window.scrollTo(0, 10000);</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled.html
new file mode 100644
index 0000000000..506ac67f0c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-huge-offset-scrolled.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="will-change-transform-huge-offset-scrolled-ref.html">
+<div style="width: 20px; height: 20px; margin-top: 10000px; background: green; will-change: transform"></div>
+<script>
+// Incrementally scroll to the bottom.
+var lastScrollY = 0;
+function scroll() {
+  window.scrollBy(0, 200);
+  if (window.scrollY > lastScrollY) {
+    lastScrollY = window.scrollY;
+    requestAnimationFrame(scroll);
+  } else {
+    document.documentElement.classList.remove('reftest-wait');
+  }
+}
+requestAnimationFrame(scroll);
+</script>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-image-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-image-ref.html
new file mode 100644
index 0000000000..d42d5b7fea
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-image-ref.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<div style="padding: 100px">
+  <div style="width: 256px; height: 256px; background: lime"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-image.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-image.html
new file mode 100644
index 0000000000..edce6c1f9b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-image.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="will-change-transform-image-ref.html">
+<img style="padding: 100px; will-change: transform" src="../../images/green-256x256.png">
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-inline-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-inline-ref.html
new file mode 100644
index 0000000000..b05c3c95ac
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-inline-ref.html
@@ -0,0 +1,18 @@
+<!doctype html>
+<title>CSS Test Reference</title>
+<style>
+  #child {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 200px;
+    height: 200px;
+    background-color: blue;
+  }
+</style>
+<div style="height: 200px"></div>
+<span id="container">
+  Child should be positioned relative to the viewport, not relative to me.
+
+  <div id="child"></div>
+</span>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-inline.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-inline.html
new file mode 100644
index 0000000000..0df497ee45
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-inline.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>will-change: transform shouldn't create a containing block if the box doesn't support transforms</title>
+<link rel=author href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel=author href="https://mozilla.org" title="Mozilla">
+<link rel=help href="https://bugzilla.mozilla.org/show_bug.cgi?id=1707170">
+<link rel=help href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel=match href="will-change-transform-inline-ref.html">
+<style>
+  #container {
+    will-change: transform;
+  }
+  #child {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 200px;
+    height: 200px;
+    background-color: blue;
+  }
+</style>
+<div style="height: 200px"></div>
+<span id="container">
+  Child should be positioned relative to the viewport, not relative to me.
+
+  <div id="child"></div>
+</span>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible-ref.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible-ref.html
new file mode 100644
index 0000000000..6eaf422d2f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible-ref.html
@@ -0,0 +1,2 @@
+<!DOCTYPE html>
+This should be visible
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible.html b/testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible.html
new file mode 100644
index 0000000000..38ddf5ec37
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-transform-zero-size-child-overflow-visible.html
@@ -0,0 +1,6 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
+<link rel="match" href="will-change-transform-zero-size-child-overflow-visible-ref.html">
+<div style="will-change: transform">
+  <div style="width: 0; height: 0; overflow: visible; white-space: nowrap">This should be visible</div>
+</div>
diff --git a/testing/web-platform/tests/css/css-will-change/will-change-will-change-1.html b/testing/web-platform/tests/css/css-will-change/will-change-will-change-1.html
new file mode 100644
index 0000000000..812dc814a0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-will-change/will-change-will-change-1.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<title>CSS Test: 'will-change: will-change' is invalid</title>
+<link rel="author" title="Cameron McCormack" href="mailto:cam@mcc.id.au">
+<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="'will-change: will-change' is invalid and thus will not induce a stacking context.">
+<style>
+div { position: absolute; }
+div div { top: 0; left: 0; width: 100px; height: 100px; }
+</style>
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div>
+  <div style="will-change: will-change;">
+    <div style="background-color: green; z-index: 2;"></div>
+  </div>
+  <div style="background-color: red; z-index: 1;"></div>
+</div>
-- 
cgit v1.2.3