See #27986 (comment):

This will override all transitions everywhere and that could lead to unexpected behaviour on custom code or css plugins. The animation is something we can't do because the spinners would freeze.

Another option is to use a PostCSS plugin to combine these rules, but AFAIK such plugin does not exist yet.

Makes sense, however to me the spinner is the only component whose animation is significant. And it could easily be fixed by displaying visually hidden text when user prefers-reduce-motion, specifically for the spinner.

So if applying this globally is not an option, there's another one I'd want to suggest for the redundancy: could we use a placeholder (extended in the transition mixins) to group transition resets?

Something like:

// In _reboot.scss, probably
@if $enable-prefers-reduced-motion-media-query {
  @media (prefers-reduced-motion: reduce) {
     %no-transition {
        transition: none !important;
     }
  }
}

// In scss/mixins/_transition.scss
@mixin transition($transition...) {
  @if $enable-transitions {
    @if length($transition) == 0 {
      transition: $transition-base;
    } @else {
      transition: $transition;
    }
  }

  @if $enable-prefers-reduced-motion-media-query {
    @extend %no-transition;
  }
}

Do you think it could solve the redundancy thing?

That could work! Feel free to test this and make a PR if you can get this to work.

i have no strong opinion here, but mention that having a single high-level animation suppression assumes that users that set prefers-reduced-motion don't want ANY animation, while this is not necessarily the case (only certain types of animations are particularly problematic/disruptive for users). the current approach allows for some nuance.

also, this general big hammer on animations will "bleed out" to any author-defined extra animations that they may have set up in their additional CSS.

Since in our test results, the @extend technique seemed to increase the bundle size, I'm gonna close this.