Fix the test count in the WPT grouping_formatter
The internal WPT API seems to have changed again, so we must revert the
previous change here to get the correct test count.
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Parse srcset attribute values
Squashed version of #18313.
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Parse sizes attribute values
Squashed version of #17808.
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Improve the media tests
This avoids a bunch of timeouts and make running the tests from `/html/semantics/embedded-content/media-elements/` in 25 seconds instead of 47 locally on my machine.
style: Cleanup the animated value setup.
We have three different enums to represent slightly different things. Reuse them
properly, and kill some code in the animated_properties module while at it.
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