Support composes at-rule #351

Problem: Composition and the Cascade

CSS Modules lets us extend classes using the composes declaration. When extending multiple classes, CSS Modules requires multiple composes declarations. Here is an example of what that CSS looks like:

.some-call-to-action {
  composes: link from 'link-styles.module.css';
  composes: button from 'button-styles.module.css';
}

The issue is that this practice of writing multiple composes declarations defies CSS Cascade (the "C" in "CSS"). Therefore, using multiple composes declarations becomes a non-preferable practice.

Only CSS declarations, that is property/value pairs, participate in the cascade.
— https://developer.mozilla.org/en-US/docs/Web/CSS/Cascade

In CSS, properties overwrite each other, and they do not combine with each other. Here is an example of the cascade:

/* global.css (earlier in order) */

li {
  margin-left: 10px;
}

/* project.css (later in order) */

li {
  margin-left: 0; /* This is a reset of `margin-left` */
}

Note: That example is used in MDN’s introduction to the CSS Cascade.

Authors may also rely on this cascade to manage browser fallbacks (e.g. .layout { display: block; display: grid }).

Even the singular example from this project’s documentation of composition relies on this expectation.

Solution: Composition At-Rule

Please consider supporting a @composes at-rule, which would be functionally identical to the current composes declaration. This would allow authors to utilize composition while still respecting the expectations inherit to CSS.

Here is an example of what that CSS would looks like, mirroring the opening example:

.some-call-to-action {
  @composes link from 'link-styles.module.css';
  @composes button from 'button-styles.module.css';
}

Additional unintentional benefits include:

• Better syntax highlighting in GitHub.
• Less conflict with stylelint declaration-block-no-duplicate-properties when using multiple composes declarations.
• Less conflict with declaration-empty-line-before when separating composes from other declarations with an empty line.
• Clearer code with stylelint-order when not separating composes from other declarations with an empty line.