[css] Targeting only Firefox with CSS

Now that Firefox Quantum 57 is out with substantial — and potentially breaking — improvements to Gecko collectively known as Stylo or Quantum CSS, you may find yourself in a situation where you have to distinguish between legacy versions of Firefox and Firefox Quantum.

From my answer here:

You can use @supports with a calc(0s) expression in conjunction with @-moz-document to test for Stylo — Gecko does not support time values in calc() expressions but Stylo does:

@-moz-document url-prefix() {
  @supports (animation: calc(0s)) {
    /* Stylo */
  }
}

Here's a proof-of-concept:

_x000D_
_x000D_
body::before {_x000D_
  content: 'Not Fx';_x000D_
}_x000D_
_x000D_
@-moz-document url-prefix() {_x000D_
  body::before {_x000D_
    content: 'Fx legacy';_x000D_
  }_x000D_
_x000D_
  @supports (animation: calc(0s)) {_x000D_
    body::before {_x000D_
      content: 'Fx Quantum';_x000D_
    }_x000D_
  }_x000D_
}
_x000D_
_x000D_
_x000D_

Targeting legacy versions of Firefox is a little tricky — if you're only interested in versions that support @supports, which is Fx 22 and up, @supports not (animation: calc(0s)) is all you need:

@-moz-document url-prefix() {
  @supports not (animation: calc(0s)) {
    /* Gecko */
  }
}

... but if you need to support even older versions, you'll need to make use of the cascade, as demonstrated in the proof-of-concept above.