Bug 204165 – Add support for selector() feature query

• Status: RESOLVED
• Resolution: DUPLICATE
• Component: CSS

[Miriam Suzanne]

- `@supports selector( <complex-selector> )`
- From CSS Conditionals Module Level 4 (https://drafts.csswg.org/css-conditional-4/)

As browsers start to roll out selectors from level 4 – :focus-within, :focus-visible, :is, :where, ::marker, extended :not and :nth-child, etc – authors are excited to start using these new tools, but we don't have the @supports control to manage fallbacks. That problem is exacerbated by the new selectors landing *before* our ability to check for them. It's frustrating to have a feature-query that has less support than the feature being queried. The longer this feature is delayed, the less useful it becomes for a new round of selectors already appearing in browsers.

Firefox has already shipped, and chromium is planning to implement this month (https://bugs.chromium.org/p/chromium/issues/detail?id=979041#c8).

[Radar WebKit Bug Importer]

<rdar://problem/57257368>

[Smoley]

*** This bug has been marked as a duplicate of bug 199237 ***