Add text and border keywords to background-clip

[frenic]

https://drafts.csswg.org/css-backgrounds-4/#background-clip

[wbamberg]

@teoli2003 , @chrisdavidmills , what's our policy for when to add new syntax? The linked spec has a big red box saying:

Not Ready For Implementation

This spec is not yet ready for implementation. It exists in this repository to record the ideas and promote discussion.

Before attempting to implement this spec, please contact the CSSWG at www-style@w3.org.

At what point in the standards process should we add new features to MDN (and mdn/data)?

[chrisdavidmills]

This something we should decide upon. In general I would say "when the feature is on a standards track and is implemented somewhere" for the earliest point we should think about documenting a standards feature on MDN. If it is implemented somewhere but is only captured in a CG note or something, then we should consider other factors such as the buzz around it.

[wbamberg]

Then I think in this case we should not add the syntax, and should close this PR. Because the spec explicitly tells us that it is not ready for implementation (i.e. not just that noone has implemented it, but noone should).

@frenic , please let me know if you need this syntax, or otherwise disagree with closing the PR.

We should have explicit guidelines for this though.

[frenic]

Yeah, some guidelines or checklists would be nice to have. So we get some kind of understanding why a certain spec is missing and when to contribute or not. The reason I added this was because we had some differences with csstype and types used in an library that was made by hand. So the issue is indirect and I really don't see any other useful reason to add it.