Carryover evaluation: CSS Pseudo-classes

[foolip]

Let's evaluate whether to continue the CSS Pseudo-classes focus area in Interop 2024.

Original proposals:

• inert attribute + :modal pseudo-class #135 by @jensimmons
• :dir() improved support #177 by @brandonmcconnell
• :user-invalid & :user-valid browser support #178 by @brandonmcconnell
• :nth-* pseudo-classes interoperability #225 by @nt1m

Tests:

• All tests
• Failing tests

We should also consider whether we should add tests for 2024.

[dbaron]

For what it's worth, I'm adding quite a few tests for :dir() as part of https://crbug.com/576815 (though unfortunately the linked WPT PRs are 3 clicks away from that -- via the different chromium-review.googlesource.com links and a comment by the export bot that links to the WPT PR), which I expect will show a bunch of bugs in other engines. There's also some ongoing related spec work, mostly to specify the behavior described in whatwg/html#3699 (comment) via whatwg/html#9554 / whatwg/html#9796 / etc. (That said, it's not super-clear how important those bugs are relative to the demand for the original feature, though it may depend on what they are.)