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[csswg-drafts] CSS mixins idea using pseudo class (#5789)

From: Mildred Ki'Lya via GitHub <sysbot+gh@w3.org>
Date: Mon, 14 Dec 2020 15:59:13 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-766658162-1607961551-sysbot+gh@w3.org>
mildred has just created a new issue for https://github.com/w3c/csswg-drafts:

== CSS mixins idea using pseudo class ==
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 https://github.com/w3c/csswg-drafts/issues/532#issuecomment-744442925 and ideas in https://github.com/w3c/csswg-drafts/issues/5624#issuecomment-744469507

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:

```css
#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:

```css
#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.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5789 using your GitHub account


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