concrete example

<selectmenu> is not part of html-tags which is used by selector-type-no-unknown rule.
It already has been renamed <selectlist> and can have a <popup> child.

ref
https://chromestatus.com/feature/5737365999976448
https://hidde.blog/custom-select-with-selectlist/
https://bugs.chromium.org/p/chromium/issues/detail?id=1121840
https://github.com/luwes/selectlist-polyfill

question

Should we have a policy that states that we have to wait for the experimentation to end before we add support for such features?
If so we would have to give some pointers on what we mean by the experimentation phase.

other examples

• Fix function-no-unknown false positive for scroll() #6537
• text-decoration-width
• selector-pseudo-class-no-unknown
  • :open and :closed
  • :top-layer
  • :popup-open
• Fix function-no-unknown false positives for anchor and anchor-size #7566