[css-highlight-api] : support Paint API for background of highlight

[nadavsinai-philips]

It would be useful to support the CSS Paint API

In our use case in Philips we wanted to use the new CSS highlight API together with the existing (draft) Paint API to be able to create custom ranges and maintain a consistent look for those which is more than color or plain background.

however the CSS Paint API does not seem to work when targeting via the ::highlight() selector

[tabatkins]

This is, unfortunately, impossible. The ::highlight pseudos, like other non-tree-abiding pseudos, restrict what properties can be used for styling with them, limiting themselves to a small list whose effects do not accidentally expose the exact node structure created by the pseudo-element. In other words, if you have some text like "<em>emphasized</em> text bar", and you highlight the text "emphasized", we don't expose whether the pseudo-element acts like it had generated the pseudo-tags around or inside the em element.

The background-image property (or any other property that takes an <image> value) would allow an author to tell which node structure was used, so we don't allow it. And the Custom Paint API works by having the paint() function be a type of <image>, so it would have similar issues.