[css-conditional-4] Reconsider disabling unforgiving parsing in `selector()`

It has been resolved to disable forgiving parsing in selector(). The initial motivation was to add styles based on user agent restrictions on :has() argument, which was previously <forgiving-relative-selector-list> but is now <relative-selector-list>.

<style>
  @supports selector(:is(undeclared|div)) { /* Forgiving :is() */
    div { color: red }
  }
  @supports selector(:not(undeclared|div)) { /* Unforgiving :not() */
    div { text-decoration: underline }
  }
</style>
<div>text</div>

I do not have a strong opinion but I find it a bit surprising that selector(:is(::before, div)) must evaluate to false whereas :is(::before, div) selects any div. If the author wants to test the support of pseudo-elements, he can write selector(::pseudo) or selector(:not(::pseudo, ::another-pseudo)).

Read the resolution in w3c/csswg-drafts issue 7280 and the CSS Conditional Rules Level 4 selector() definition linked here. Review the outdated interoperability example only to understand the original concern; this issue is done only if the specification's parsing behavior has an agreed resolution.