Bug 1860907 [wpt PR 42732] - Implement updated HTML language inherita…

…nce rules., a=testonly

Automatic update from web-platform-tests
Implement updated HTML language inheritance rules.

This makes two changes to match the HTML spec:
* only honor the unnamespaced lang attribute on HTML and SVG elements
* have <slot> elements in a shadow tree inherit language from the shadow
  host

Note that an update to the HTML specification regarding the SVG elements
case is proposed in whatwg/html#9882, but
https://www.w3.org/TR/SVG/struct.html#LangAttribute already defines it.

These changes match the related changes to the dir attribute, but use a
separate feature flag as a killswitch, since they are separable.

Fixed: 1490711
Bug: 576815
Change-Id: I6462181e6d4433aa1dcc9a2c42e5c12754830e52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4974775
Commit-Queue: Di Zhang <dizhanggchromium.org>
Commit-Queue: David Baron <dbaronchromium.org>
Reviewed-by: Di Zhang <dizhanggchromium.org>
Cr-Commit-Position: refs/heads/main{#1215017}

--

wpt-commits: 6148c29da568184fae4184f739ecccaafd651af6
wpt-pr: 42732

UltraBlame original commit: 6425b442dc96bbae14adf5799ce58ea64fcfd493

testing/web-platform/tests/html/dom/elements/global-attributes/lang-attribute.window.js

@@ -0,0 +1,204 @@
+test
+(
+(
+)
+=
+>
+{
+const
+container
+=
+document
+.
+createElement
+(
+"
+div
+"
+)
+;
+document
+.
+body
+.
+append
+(
+container
+)
+;
+container
+.
+setAttribute
+(
+"
+lang
+"
+"
+en
+-
+CA
+"
+)
+;
+const
+child
+=
+document
+.
+createElementNS
+(
+"
+div
+"
+"
+test
+"
+)
+;
+container
+.
+append
+(
+child
+)
+;
+child
+.
+setAttribute
+(
+"
+lang
+"
+"
+en
+-
+NZ
+"
+)
+;
+assert_true
+(
+container
+.
+matches
+(
+"
+:
+lang
+(
+en
+-
+CA
+)
+"
+)
+"
+container
+matches
+en
+-
+CA
+"
+)
+;
+assert_true
+(
+child
+.
+matches
+(
+"
+:
+lang
+(
+en
+-
+CA
+)
+"
+)
+"
+child
+matches
+en
+-
+CA
+"
+)
+;
+assert_false
+(
+container
+.
+matches
+(
+"
+:
+lang
+(
+en
+-
+NZ
+)
+"
+)
+"
+container
+does
+not
+match
+en
+-
+NZ
+"
+)
+;
+assert_false
+(
+child
+.
+matches
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+"
+:
+lang
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+en
+-
+NZ
+)
+"
+)
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+child
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+-
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+"
+)
+;
+container
+.
+remove
+(
+)
+;
+}
+"
+unnamespaced
+lang
+attribute
+only
+works
+on
+elements
+in
+the
+HTML
+namespace
+"
+)
+;