[css-scroll-anchoring-1] Explicit anchoring

The Scroll Anchoring module already allows to opt-out elements of the scroll anchoring algorithm via overflow-anchor: none;. And auto makes an element potentially eligible for anchoring. Though there is currently no way to enforce anchoring to an element.

The idea behind this is to allow the author to tell the user agent explicitly that it should prefer specific elements when running the anchor node selection algorithm. A use case for this are several content pieces, of which one is the main content.

To make this possible, there should be a new value like prefer for the overflow-anchor property.

Elements having this value set should be checked first by influencing the order in which the elements are checked in step 2 of the anchoring node selection algorithm.
It might also influence the candidate examination algorithm by preferring those elements over fully visible elements even when they are just partially visible.

Sebastian

Start with the Scroll Anchoring module, the overflow-anchor property definition, and the anchoring node selection algorithm referenced in the issue. Determine the specification changes needed for a prefer value and its selection behavior; done means the proposal is resolved in the draft with agreed algorithm text.