PostCSS CSS Variables

PostCSS plugin to transform CSS Custom Properties (CSS variables) syntax into a static representation. This plugin provides a future-proof way of using most of CSS variables features, including selector cascading with some caveats, because this can only see the CSS, not the potentially dynamic HTML and DOM the CSS is applied to.

Install

npm install postcss-css-variables --save-dev

Table of Contents

• Code Playground
• Usage
• Syntax
  • Defining Custom Properties with --*
  • Using Variables/Custom Properties with var()
• Features
  • At-rules like @media, @support, etc.
  • Pseudo-classes and Elements
  • Nested Rules
  • calc()
• Why?
  • Interoperability
  • Differences from postcss-custom-properties
• Caveats
• Options
• Quick Reference/Notes
• Testing
• Changelog

Code Playground

Try it in the playground and see what you think! Just add some CSS and see to see the final transformed/compiled CSS. You can try anything here in the playground, too.

Usage

For more general PostCSS usage, look here.

var postcss = require('postcss');
var cssvariables = require('postcss-css-variables');

var fs = require('fs');

var mycss = fs.readFileSync('input.css', 'utf8');

// Process your CSS with postcss-css-variables
var output = postcss([
		cssvariables(/*options*/)
	])
	.process(mycss)
	.css;

console.log(output);

Syntax

Defining Custom Properties with --*

A custom property is any property whose name starts with two dashes --. A property must be in a rule.

Note: :root is nothing more than the selector for the root DOM node. Any other selector like .class, #id, or even #foo ~ .bar > span.baz works.

:root {
	--foo-width: 100px;
	--foo-bg-color: rgba(255, 0, 0, 0.85);
}

.foo {
	--foo-width: 100px;
	--foo-bg-color: rgba(255, 0, 0, 0.85);
}

Custom properties can be declared multiple times, but like variable scope in other languages, only the most specific one takes precedence.

:root {
    --some-color: red;
}

.foo {
    /* red */
    color: var(--some-color);
}


.bar {
    --some-color: blue;
    /* blue */
    color: var(--some-color);
}

.bar:hover {
    --some-color: green;
    /* Automically gets a `color: green;` declaration because we `--some-color` used within scope elsewhere */
}

W3C Draft: CSS Custom Properties for Cascading Variables, section 2

Using Variables/Custom Properties with var()

.foo {
	width: var(--foo-width);
	/* You can even provide a fallback */
	background: var(--foo-bg-color, #ff0000);
}

W3C Draft: CSS Custom Properties for Cascading Variables, section 3

Features

At-rules like @media, @support, etc.

It's perfectly okay to declare CSS variables inside media queries and the like. It'll work just as you would expect.

:root {
	--width: 100px;
}

@media (max-width: 1000px) {
	:root {
		--width: 200px;
	}
}

.box {
	width: var(--width);
}

Will be transformed to:

.box {
	width: 100px;
}

@media (max-width: 1000px) {
	.box {
		width: 200px;
	}
}

Pseudo-classes and Elements

Psuedo-classes are also dealt with correctly, because it's easy to statically determine.

.foo {
	--foo-color: red;
	color: var(--foo-color);
}

.foo:hover {
	--foo-color: green;
}

Will be transformed to:

.foo {
	color: red;
}

.foo:hover {
	color: green;
}

Nested Rules

This pairs very well with postcss-nested or postcss-nesting, adding support for nested rules. For either, you must put the plugin before postcss-css-variables in the plugin stack so that the & references are expanded first (postcss-css-variables doesn't understand them). For example, with postcss-nested, your PostCSS setup would look like this:

var postcss = require('postcss');
var cssvariables = require('postcss-css-variables');
var nested = require('postcss-nested');

var fs = require('fs');

var mycss = fs.readFileSync('input.css', 'utf8');

var output = postcss([
		// Flatten/unnest rules
		nested,
		// Then process any CSS variables
		cssvariables(/*options*/)
	])
	.process(mycss)
	.css;

console.log(output);

For a simple example with nesting:

.box-foo {
	--some-width: 150px;
	width: var(--some-width);

	.box-bar {
		width: var(--some-width);
	}
}

With also postcss-nesting, this will be transformed to:

.box-foo {
	width: 150px;
}

.box-foo .box-bar {
	width: 150px;
}

For a more complex example with a media query:

:root {
	--some-width: 150px;
}

.box-foo {
	width: var(--some-width);

	.box-bar {
		width: var(--some-width);
	}
}

@media (max-width: 800px) {
	.box-foo {
		--some-width: 300px;
	}
}

Will be transformed to:

.box-foo {
	width: 150px;
}

.box-foo .box-bar {
	width: 150px;
}

@media (max-width: 800px) {
	.box-foo {
		width: 300px;
	}
	
	.box-foo .box-bar {
		width: 300px;
	}
}

Why?

This plugin was spawned out of a discussion on the cssnext repo and a personal need.

There is another similar plugin available, postcss-custom-properties, although it restricts itself much more than this plugin, preferring partial spec conformance. This plugin has the same capabilities but also adds imperfect feature support which stem from not being to know what the DOM will look like when you compile your CSS. We instead look at the explicit structure of your CSS selectors.

Interoperability

Putting postcss-css-variables in place of postcss-custom-properties should work out of the box.

In postcss-custom-properties, CSS variable declarations are specifically restricted to the :root selector. In postcss-css-variables, this is not the case and they may be declared inside any rule with whatever selector. The variables are substituted based on statically known CSS selector inheritance.

Differences from postcss-custom-properties

In postcss-custom-properties, CSS variable declarations are specifically restricted to the :root selector. In postcss-css-variables, this is not the case and they may be declared inside any rule with whatever selector.

Here's a quick overview of the differences:

• CSS variables may be declared in any selector like .foo or .foo .bar:hover, and is not limited to just :root
• CSS variables may be declared in @media, @support, and other at-rules.
• CSS variables may be declared in :hover and other psuedo-classes, which get expanded properly.
• Variables in nested rules can be deduced with the help of postcss-nested or postcss-nesting.

Continue to the next section to see where some of these might be unsafe to do. There are reasons behind the ethos of why the other plugin, postcss-custom-properties, is very limited in what it supports, due to differing opinions on what is okay to support.

Caveats

When you declare a CSS variable inside one selector, but consume it in another, this does make an unsafe assumption about it which can be non-conforming in certain edge cases. Here is an example where these limitations result in non-conforming behavior.

Using the following CSS:

.component {
  --text-color: blue;
}

.component .header {
  color: var(--text-color);
}

.component .text {
  --text-color: green;
  color: var(--text-color);
}

Note the nested markup below. We only know about the DOM's inheritance from your CSS selectors. If you want nest multiple times, you need to be explicit about it in your CSS which isn't necessary with browser that natively support CSS variables. See the innermost <h1 class="title">

<div class="component">
    Black

    <h1 class="title">
        Blue

        <p class="decoration">
            Green

            <h1 class="title">Blue with this plugin, but green per spec</h1>
        </p>
    </h1>
</div>

postcss-custom-properties avoids this problem entirely by restricting itself to just the :root selector. This is because the developers there would prefer to not support a feature instead of something almost-spec-compliant like what postcss-css-variables does.

Options

preserve (default: false)

Allows you to preserve custom properties & var() usage in output.

Possible values:

• false: Removes --var declarations and replaces var() with their resolved/computed values.
• true: Keeps var() declarations in the output and has the computed value as a fallback declaration. Also keeps computed --var declarations.
• 'computed': Keeps computed --var declarations in the output. Handy to make them available to your JavaScript.

variables (default: {})

Define an object map of variables in JavaScript that will be declared at the :root scope.

Can be a simple key-value pair or an object with a value property and an optional isImportant bool property.

The object keys are automatically prefixed with -- (according to CSS custom property syntax) if you do not provide it.

var postcss = require('postcss');
var cssvariables = require('postcss-css-variables');

postcss([
	cssvariables({
		variables: {
			'--some-var': '100px',
			'--other-var': {
				value: '#00ff00'
			},
			'--important-var': {
				value: '#ff0000',
				isImportant: true
			}
		}
	})
])
.process(css, opts);

Quick Reference/Notes

• This plugin was spawned out of a discussion on the cssnext repo
• We provide a larger CSS variable feature subset than postcss-custom-properties.
• Related links and issues:
  • var declared in media query should pull in properties that use/reference that var on cssnext/cssnext
  • Investigate support for media-query scoped properties on postcss/postcss-custom-properties
  • remove :root limitation by injecting rules with new declarations that just contains modified properties. on postcss/postcss-custom-properties

Testing

We have a suite of Mocha tests. If you see something that doesn't have coverage, make an issue or pull request.

Run once:

npm install

Run whenever you want to test:

npm run test