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