[css-values] Define mix() with <declaration-value> #8096
Description
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