Opacity for color-mix is not correctly pre-multiplied. #1195

@keithamus The current implementation has a bug, see also: https://bugzilla.mozilla.org/show_bug.cgi?id=1901991

The implementation of csskit appears to have the same bug as Firefox; perhaps they can be fixed together.

input

div {
  background-color: color-mix(
    in srgb,
    color(srgb 0.4 0.4 0 / none),
    color(srgb 0.4 0 0.4 / 0.5)
  );
}

.legacy {
  background-color: color-mix(
    in srgb,
    rgb(100 100 0 / none),
    rgb(100 0 100 / 0.5)
  );
}

output

div{background-color:#66006640}.legacy{background-color:#64006440}

expected

div{background-color:#66333380}.legacy{background-color:#64323280}

playground

Originally posted by @yisibl in #871