I'm bullish on CSS mixins

As a component library developer, @mixin from the Functions and Mixins Module has quickly become one of my favorite parts of CSS. Even though I have to polyfill it, mixins solve so many of the problems with classes+variables:

• Classes make a non-Tailwind project feel like Tailwind. Mixins go with the rest of your styles, upholding separation of concerns.
• Classes can't be toggled from CSS. I literally can't do "shrink the label when the input is active" - I have to either copy the class's contents in (and hope I remember to update it) or use something arbitrary like CSS Modules that doesn't work well with Svelte. Mixins can be @applyd at any time!
• You can't customize with pure classes because their styles are sometimes inlined, so you have to do something ugly and space consuming like:

  :root {
    --m3-font: Roboto, system-ui;
    --m3-font-label: var(--m3-font);
    --m3-font-label-large-size: 0.875rem;
    --m3-font-label-large-height: 1.429;
    --m3-font-label-large-tracking: 0.006rem;
    --m3-font-label-large-weight: 500;
  }
  .m3-font-label-large,
  .m3-font-label-medium,
  .m3-font-label-small {
    font-family: var(--m3-font-label);
  }
  .m3-font-label-large {
    font-size: var(--m3-font-label-large-size);
    line-height: var(--m3-font-label-large-height);
    letter-spacing: var(--m3-font-label-large-tracking);
    font-weight: var(--m3-font-label-large-weight);
  }

  With mixins, the user can just define their own @mixin --m3-label-large!

Mixins could kill or reform CSS Modules

However: my polyfill can't transform dynamic access like size="--m3-button-large" color="--m3-primary", and even once mixins are widely supported, the ecosystem won't be able to tree shake dynamic access. We need a .mixin.css or something that you can import just what you need from, so unused mixins can be eliminated - or more generically, we need support for granular imports from a CSS file.

We would also need syntax. For example, Svelte would need something like style|apply={[size, color]}.