[css-images-4] [css-color-5] - [color-interpolation-method] should allow custom idents

[argyleink]

Given Color 5 allows loading of custom color profiles, those loaded spaces should be available to use in CSS Images 4 with gradients like:

@color-profile --myspace {
  src: url(https://myspace.colors/imaginary-path.icc);
}

.card {
  color: color(--myspace 200 50 50);
  background: linear-gradient(to bottom right in --myspace, 
    color(--myspace 200 50 50), 
    color(--myspace 100 50 50)
  );
}

or even

.card {
  background: linear-gradient(in --myspace, lime, blue);
}

[svgeesus]

(also tagging color 4 which defines <color-interpolation-method>, although we should probably extend it in Color 5 and leave 4 alone -on second thought yes, that makes more sense)

[LeaVerou]

Yes, this should be Color 5. It doesn't make sense to allow dashed idents in <color-interpolation-method> when there are no color spaces using them.

[svgeesus]

<color-interpolation-method> now extended in CSS Color 5 to add custom-colorspace> which is a <dashed-ident> and must have been declared in a @color-profile rule.