[css-view-transitions-2] Revisit the -ua- types fix to make parsing invalid

[noamr]

No description provided.

[cdoublev]

That works for me. It is no longer ambiguous. Thanks!

The requirement to exclude some identifiers from matching <custom-ident> is phrased in various ways across the specs (some examples below), with some inconsistencies like not mentioning ASCII case-insensitivity or excluding identifiers that are already specified as reserved by CSS Values:

When parsing positionally-ambiguous keywords in a property value, a <custom-ident> production can only claim the keyword if no other unfulfilled production can claim it.

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<animateable-feature> = scroll-position | contents | <custom-ident>

The <custom-ident> production used here excludes the keywords will-change, none, all, auto, scroll-position, and contents, in addition to the keywords normally excluded from <custom-ident>.

<container-name> = <custom-ident>

The keywords none, and, not, and or are excluded from this <custom-ident>.

<counter-name> = <custom-ident>

A <counter-name> name cannot match the keyword none; such an identifier is invalid as a <counter-name>.

<counter-style-name> = <custom-ident>

A <counter-style-name> is a <custom-ident> that is not an ASCII case-insensitive match for none.