[css-values] Define mix() with <declaration-value>

[Loirooriol]

https://drafts.csswg.org/css-values-4/#interpolate

mix( <percentage> ';' <start-value> ';' <end-value> )

• <start-value>
  The value at the “start” (0%) of the interpolation range.
• <end-value>
  The value at the “end” (100%) of the interpolation range.

It doesn't say what's the grammar for <start-value> and <end-value>.

I think they should be defined to be equivalent to <declaration-value>, or just use <declaration-value> directly.

Related: #8091

[cdoublev]

Aside:

I think w3c/reffy is not able to extract its production rule because its syntax does not have the usual structure <name> = rule, ie:

mix( <percentage> ';' <start-value> ';' <end-value> )

instead of:

<mix()> = mix( <percentage> ';' <start-value> ';' <end-value> )

Do you want this simplified form to be recognized by w3c/reffy?

[tabatkins]

I think they should be defined to be equivalent to <declaration-value>

Yes.

Do you want this simplified form to be recognized by w3c/reffy?

No, we should fix the markup.