CSS mixins idea using pseudo class

[mildred]

I have been searching through issues to get an idea in how css mixins could be implemented in a future CSS specification. The goal is to avoid having to add class names to elements from CSS frameworks, but instead use semantic selectors and assign a style for each element.

In short, replace a danger class that puts the element in red with a CSS mixin (whatever the syntax) that can pull the red color property from elsewhere. I have been looking in particular at the @apply rule and I still have open questions about it in #532 (comment) and ideas in #5624 (comment)

If the idea is to keep the way CSS is structured right now, conditionals at the top level with @supports and @media, selectors below and then properties, mixin matching could be done at the selector level using a pseudo-class. For example:

#my-error-message {
  mixin: danger message;
}

:mixin(danger) {
  background-color: red;
}

div:mixin(message) {
  border: thin black solid;
}

span:mixin(message) {
  text-decoration: underline;
}

This is pretty self-explaining, the mixin CSS property defines a list of words that can be then matched by the :mixin() pseudo-class.

I first imagined that any property could be matched and using custom properties to achieve similar result:

#my-error-message {
  --mixin-danger: on;
  --mixin-message: on;
}

:defines(--mixin-danger: on) {
  background-color: red;
}

:defines(--mixin-message: on) {
  border: thin black solid;
}

A problem with that is that the custom property is inherited by default and it would be logical for :defines(--mixin-danger: on) to match #my-error-message and its descendants. For mixins to be usable we need to avoid matching descendants.

[LeaVerou]

Unfortunately, using pseudo-classes is not an option, they are resolved at a totally different stage and changing that would be very problematic architecturally. @tabatkins or @emilio can elaborate.

[plehegar]

(as a reminder, *-needs-resolution use is reserved for horizontal groups):
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