[css-view-transitions-2] Require one or more custom ident for view-transition-class and type

[cdoublev]

The value definitions of view-transition-class and type currently allow an empty value with <custom-ident>*, which means zero or more <custom-ident>:

Name: view-transition-class
Value: none | <custom-ident>*

See this discussion about accepting an empty value in the property value range.

[noamr]

Hi, do you mind rebasing? I'll be happy to merge it in (for classes, types are already sorted)

[noamr]

@tabatkins mind marking this as non-substantive for IPR?

[w3cbot]

astearns marked as non substantive for IPR from ash-nazg.

[astearns]

@cdoublev could you contact me? (email is in my profile, or on irc, or whatever else)

I’d like to see if we can remove the constant IPR checks we have to deal with on your PRs