I presumed that the anchor() function is added as a new type to insets e.g. auto | <length-percentage> | <anchor()>.

However, the validity section mentions that the anchor function is valid if, among other things, is used in inset properties, and that if it's invalid, it will resolve to its fallback value (If provided). This could be read as <anchor()> being part of <length-percentage>, I think.

So does it mean something like width: anchor(--foo left, 10px); will resolve to width: 10px or width: initial? Could see this happening through unregistered properties/trying to use these functions outside of the applicable properties (e.g. #9827)

Same question applies to anchor-size() applied outside of sizing/margin/inset properties.