Update the `open` variant to addtionally match the new `:open` pseudo
class.
See spec at https://drafts.csswg.org/selectors-4/#open-state
See Chromium intent to ship:
https://groups.google.com/a/chromium.org/g/blink-dev/c/Pdo4oOXYKcA
`:open` matches open `<details>` and `<dialog>` elements (already
covered by [open]), but also matches select elements with their picker
open and inputs with their picker open.
Thanks to `:is()` taking a forgiving selector list, this is safe to add
even with limited browser support.
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>
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