[css-anchor-position] anchor() Fallback type inconsistency vs WPT

[dshin-moz]

For the anchor() function's optional fallback value, the spec text specifies <length-percentage>, but the corresponding WPT, anchor-parse-valid.html, considers the use of anchor() function itself to be valid, seemingly contradicting the spec text.

It seems to me that both the spec and the test were initially committed this way - Which one is correct?

[emilio]

The spec seems clear, unless there's a strong cause to allow anchor() in the fallback?

cc @tabatkins @fantasai

[dshin-moz]

Also the case for anchor-size() - Fallback on spec is <length-percentage>, but WPT allows anchor-size().

[bfgeek]

So the fallback is somewhat desirable IMO, e.g. you can chain things together like:

right: anchor(--a left, anchor(--b left))

e.g. if "a" doesn't exist, use "b".

Note this is different than something like:
min(anchor(--a left), anchor(--b left)) as this will resolve the unknowns to zero, having an affect on the result.

[dshin-moz]

Hm, probably can draw an analogy to var() allowing multiple fallbacks through nesting..