UA stylesheets in CSS specs cause interop issues

For the record, other UA stylesheets defined include:

• CSS Color Adjustment Module
• CSS Counter Styles
• CSS Generated Content for Paged Media (informative)
• CSS Inline Layout
• CSS Lists and Counters (informative)
• CSS Pseudo-Elements (SHOULD)
• CSS Ruby Annotation (informative)
• CSS Text (informative)
• CSS Text Decoration (informative)
• CSS Transforms has an "initial used value is 0 0 as if > the user agent style sheet contained"(!!!)
• CSS Basic User Interface
• CSS Writing Modes
• CSS 2 (informative)
• Media Queries

Originally posted by @gsnedders in https://github.com/w3c/csswg-drafts/issues/8573#issuecomment-1466259733

In https://bugzilla.mozilla.org/show_bug.cgi?id=645642 match-parent was implemented in Gecko, and per https://bugzilla.mozilla.org/show_bug.cgi?id=645642#c14 the suggested UA stylesheet for option was implemented. But it was not implemented in WebKit or Chromium.

https://drafts.csswg.org/css-text/#default-stylesheet says

This appendix is informative, and is to help UA developers to implement a default stylesheet for HTML, but UA developers are free to ignore or modify as appropriate.

I do not find these optional UA stylesheets helpful at all, and would prefer if the UA stylesheet for HTML was left entirely to the HTML standard to define.

Maybe there are exceptional cases where a new feature isn't ready to be included in HTML's UA stylesheet. If you want to include example UA stylesheets, please say that they must not be implemented in UAs, so that they don't cause further interop issues.

Review the UA stylesheet sections linked in the issue, including CSS Color Adjustment, CSS Text, CSS Transforms, CSS UI, CSS Writing Modes, and CSS 2. Compare their guidance with the HTML UA stylesheet approach and the cited Gecko, WebKit, and Chromium context. Done means the affected specifications have an agreed, consistent policy that avoids creating interop discrepancies.