PostCSS Custom Properties Fallback

This plugins adds fallbacks to your CSS Custom Properties and works well as a compantion to PostCSS Custom Properties.

Pop Quiz!

If we remove --color from :root, what color will h1 have in modern browsers?

:root {
-  --color: red;
}

body {
  color: green;
}

h1 {
  color: red;
  color: var(--color);
}

Red or green, expand the right answer (no cheating/googling!):

h1 is red

Nope, it's green!

Intuitively it's easy to think that if --color isn't defined, then the browser should skip the color: var(--color) and use the valid color: red above it. Especially since this is what happens in older browsers that don't support CSS Custom Properties.

The right answer is to use the second argument in var() (see Example 10 in the spec), also known as the fallback argument:

color: var(--color, red);

Now it works like expected. See the spec for more information on how invalid/missing values are treated.

h1 is green

Right answer! Check the wrong answer to learn why that is.

Usage

Add PostCSS Custom Properties Fallback to your project:

npm install postcss-custom-properties-fallback --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssCustomPropertiesFallback = require('postcss-custom-properties-fallback');

postcss([postcssCustomPropertiesFallback(/* pluginOptions */)]).process(
  YOUR_CSS /*, processOptions */
);

Options

importFrom

The importFrom option is required. It works like from CSS Custom Properties, except it doesn't support importing from CSS yet.

postcssCustomPropertiesFallback({
  importFrom: { customProperties: { '--color': 'red' } },
});

h1 {
  color: var(--color);
}

/* becomes */

h1 {
  color: var(--color, red);
}