[CSS] image-set() support

[chrisdavidmills]

Acceptance criteria

1. • The listed features are documented sufficiently on MDN
2. • BCD is updated
3. • Interactive example and data repos are updated if appropriate
4. • The content has been reviewed as needed

Features to document

1. image-set() as a value for content and cursor properties.
2. image-set() function in general

Related Gecko bugs

1. • Support image-set() on the cursor property. Probably just needs an example adding to the cursor page, rel note and BCD?
2. • Support image-set() on the content property. Same as above, but for the content property?
3. • Enable layout.css.image-set.enabled in all channels.. Update image-set() page and BCD, add rel note?

For folks helping with Firefox-related documentation features — make sure above AC have been done, but also:

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
• Add entry to Firefox experimental features page if feature is not yet enabled in release

[rachelandrew]

@chrisdavidmills This isn't actually in the spec yet for cursor (although it does say that UAs can support image) w3c/csswg-drafts#5831

So we probably don't want to add too much to the page right now. I'm wondering about BCD for cursor also? Should I add a line about support for <image>? As that is really where image-set() comes from, this is a change to allow the cursor property to support <image>, the but of implementation in 88 is adding the support for image-set to cursor.

[rachelandrew]

• content I have updated the page and checked that the syntax correctly includes image-set() it already did
• for cursor there is still a lot of discussion on this [css-ui-4] Added image-set() function to 'cursor' property w3c/csswg-drafts#6153 so I think it is probably premature to do too much in the way of documentation or BCD. I'm not sure it really needs much anyway as image-set being supported is the main thing, we only really need to document it on cursor if it behaves differently.
• BCD is updated for image-set()
• Release notes are done
• Updated the three Bugzilla bugs

[chrisdavidmills]

This one looks like it can be closed too.