[css-fonts-5] Allow to define palette colors outside of `@font-palette-values`

[SebastianZ]

Thinking about the proposal for animating font-palette in #8922, the question I had in mind was, why not allow to animate the different colors individually?

The main benefit of that is that you can transition each color individually and with that also define the interpolation color space. And besides the transition/animation use case, you'd be able to change each color on usage. That would require a new property to access and overwrite the colors outside of @font-palette-values, e.g. called font-palette-color.

Syntax suggestion:

font-palette-color: [ <integer [0,∞]> [ <absolute-color-base> | from-palette ] ]#

So it would basically work like the override-colors descriptor descriptor but may also use the color from the palette via the from-palette keyword.

Example for an animation:

@keyframes --palette-animation {
  from {
    font-palette-color:
      1 red,
      2 from-palette
      3 oklch(0.63 0.12 105.7);
  }
  to {
    font-palette-color:
      1 blue,
      2 hsl(80 90% 60%),
      3 oklch(0.5 0.90 185.7);
  }
}

One open question is what to do with colors not defined in font-palette-color and font-palette itself is not animated. Possible solutions I can think of:

1. Use the colors from the palettes and
   1. interpolate them like other colors
   2. interpolate them discretely
2. Use a default color like currentcolor (that might also be overwritten by allowing to define a color without index, e.g. font-palette-color: black, 2 blue; would set the second color to blue and the rest of the colors in the palette to black)

Initially, I wanted to add that as a comment to #8922, though as it covers additional use cases besides animation and is orthogonal to the proposal there, I thought it would be worth to discuss this separately.

Sebastian

[drott]

If we would start from scratch and there would be no existing font-palette property, I agree this is a viable proposal.

But since we have existing content and and existing definition offont-palette I find this proposal lacks in compatibility. We would define a new font-palette-color property that partially duplicates behavior of the existing font-palette property and has a complicated interaction with it when both would be present.

Compared to using references to the @font-palette-values rules, the property becomes very verbose, and needs repeating the full palette color set text over the full set of palette colors (up to 65k of them) if the definition differs from the font. IMO the verbosity would not be justified as control over each individual color from the palette is likely a less frequent use case compared to switching between fixed and reusable full palette definitions in @-rules.