PostCSS plugin enabling custom property sets references

Refer to postcss-custom-properties for DOMless limitations.

Spec (editor's draft): https://tabatkins.github.io/specs/css-apply-rule
Browser support: https://www.chromestatus.com/feature/5753701012602880

⚠️ The @apply rule and custom property sets most likely won't get any more support from browser vendors as the spec is yet considered deprecated and alternative solutions are being discussed.
Refer to following links for more infos:

• https://discourse.wicg.io/t/needed-new-champion-for-css-apply-rule/2012
• WICG/webcomponents#300 (comment)
• https://www.xanthir.com/b4o00
• w3c/csswg-drafts#1047
• https://chromium.googlesource.com/chromium/src/+/5874fca7324e4523a4bdecc8999bdadfdb6c4eff

npm install postcss-apply --save-dev

const fs = require('fs');
const postcss = require('postcss');
const apply = require('postcss-apply');
let input = fs.readFileSync('input.css', 'utf8');
postcss()
  .use(apply)
  .process(input)
  .then(result => {
    fs.writeFileSync('output.css', result.css);
  });

/* input */
:root {
  --toolbar-theme: {
    background-color: rebeccapurple;
    color: white;
    border: 1px solid green;
  };
}
.Toolbar {
  @apply --toolbar-theme;
}

/* output */
.Toolbar {
  background-color: rebeccapurple;
  color: white;
  border: 1px solid green;
}

const themes = {
  /* Set names won't be transformed, just `--` will be prepended. */
  'toolbar-theme': {
    /* Declaration properties can either be camel or kebab case. */
    backgroundColor: 'rebeccapurple',
    color: 'white',
    border: '1px solid green',
  },
};
[...]
postcss().use(apply({ sets: themes }))
[...]

/* input */
.Toolbar {
  @apply --toolbar-theme;
}

/* output */
.Toolbar {
  background-color: rebeccapurple;
  color: white;
  border: 1px solid green;
}

type: Boolean
default: false
Allows for keeping resolved declarations and @apply rules alongside.

type: { [customPropertyName: string]: Object | string }
default: {}
Allows you to pass an object or string of custom property sets for :root. These definitions will be prepended, in such overridden by the one declared in CSS if they share the same name. The keys are automatically prefixed with the CSS -- to make it easier to share sets in your codebase.

• Pascal Duez

postcss-apply is unlicensed.