Update background-clip:text status, add background-clip:border

[SelenIT]

Summary

Mark background-clip:text as experimental (since it's only in the CSS Backgrounds 4 editor's draft) and mark Firefox's implementation as experimental because of many open issues related to it.

Add background-clip:border from the same spec for consistency.

Test results and supporting details

None

Related issues

Fixes #23943

[SelenIT]

The auto tests script states that "Experimental should be set to false as the feature is supported in multiple browser engines" and marks experimental status for background-clip:text as "Error", but I believe this particular case is a valid exception from this rule, as the "should" keyword is supposed to allow.

The text value for background-clip property was unilaterally added by WebKit as a non-standard extension and became widely supported only because WebKit and its derivative Blink engines became so popular. The CSSWG has opposed standardizing this feature for years, it was never properly specified. Firefox's implementation still lacks some important use cases like multi-layer backgrounds combining text with other background-clip values for other layers. Since this value is in the spec draft now, it can't be marked as non-standard anymore, but marking it non-experimental only because of popularity of the clones of the engine that introduced this experimental feature doesn't feel right.

I would suggest keeping its experimental status until the CSS Backgrounds Level 4 spec matures and/or Firefox's implementation issues are fixed, if it's possible.

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.