What is postcss-custom-properties?
The postcss-custom-properties npm package is a PostCSS plugin that allows you to use CSS Custom Properties (also known as CSS variables) in environments that do not support them natively. It transforms CSS variables into static values based on your configurations, making it easier to maintain themes and styles dynamically across your project.
What are postcss-custom-properties's main functionalities?
Transform CSS Custom Properties
This feature allows the transformation of CSS custom properties into their corresponding static values. It is useful for supporting older browsers that do not understand CSS variables.
/* Input CSS */
:root {
--main-color: red;
}
a {
color: var(--main-color);
}
/* Output CSS */
a {
color: red;
}
Preserve option
With the preserve option set to true, the plugin outputs both the transformed static value and the original variable. This is useful for progressive enhancement.
/* Input CSS */
:root {
--main-color: red;
}
a {
color: var(--main-color);
}
/* Output CSS with preserve: true */
a {
color: red;
color: var(--main-color);
}
Other packages similar to postcss-custom-properties
cssnext
cssnext is a PostCSS plugin that allows you to use future CSS features today. It includes support for CSS custom properties among other features. Compared to postcss-custom-properties, cssnext offers a broader range of CSS features but might be heavier due to its comprehensive nature.
postcss-preset-env
postcss-preset-env lets you convert modern CSS into something most browsers can understand, determining the polyfills you need based on your targeted browsers or runtime environments. It includes handling of CSS custom properties as part of its feature set, similar to postcss-custom-properties, but is more configurable and includes various stages of CSS specifications.
PostCSS Custom Properties 
npm install postcss-custom-properties --save-dev
PostCSS Custom Properties lets you use Custom Properties in CSS, following
the CSS Custom Properties specification.

:root {
--color-blue-dark: rgb(0, 61, 184);
--color-blue-light: rgb(0, 217, 255);
--color-pink: rgb(255, 192, 211);
--text-color: var(--color-pink);
}
.element {
/* custom props */
--border-color: var(--color-blue-light);
/* props */
border: 1px solid var(--border-color);
color: var(--text-color);
}
.element--dark {
--border-color: var(--color-blue-dark);
}
/* becomes */
:root {
--color-blue-dark: rgb(0, 61, 184);
--color-blue-light: rgb(0, 217, 255);
--color-pink: rgb(255, 192, 211);
--text-color: var(--color-pink);
}
.element {
/* custom props */
--border-color: var(--color-blue-light);
/* props */
border: 1px solid rgb(0, 217, 255);
border: 1px solid var(--border-color);
color: rgb(255, 192, 211);
color: var(--text-color);
}
.element--dark {
--border-color: var(--color-blue-dark);
}
Note:
- Only processes variables that were defined in the
:root
or html
selector.
- Locally defined variables will be used as fallbacks only within the same rule, but not elsewhere.
- Fallback values in
var()
will be used if the variable was not defined in the :root
or html
selector.
Usage
Add PostCSS Custom Properties to your project:
npm install postcss postcss-custom-properties --save-dev
Use it as a PostCSS plugin:
const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');
postcss([
postcssCustomProperties()
]).process(YOUR_CSS );
Options
preserve
The preserve
option determines whether properties using
custom properties should be preserved in their original form. By default these are preserved.
Custom property declarations are always preserved only var()
functions can be omitted.
postcssCustomProperties({ preserve: false })
:root {
--color-blue-dark: rgb(0, 61, 184);
--color-blue-light: rgb(0, 217, 255);
--color-pink: rgb(255, 192, 211);
--text-color: var(--color-pink);
}
.element {
/* custom props */
--border-color: var(--color-blue-light);
/* props */
border: 1px solid var(--border-color);
color: var(--text-color);
}
.element--dark {
--border-color: var(--color-blue-dark);
}
/* becomes */
:root {
--color-blue-dark: rgb(0, 61, 184);
--color-blue-light: rgb(0, 217, 255);
--color-pink: rgb(255, 192, 211);
--text-color: var(--color-pink);
}
.element {
/* custom props */
--border-color: var(--color-blue-light);
/* props */
border: 1px solid var(--border-color);
color: rgb(255, 192, 211);
}
.element--dark {
--border-color: var(--color-blue-dark);
}
Modular CSS Processing
If you're using Modular CSS such as, CSS Modules, postcss-loader
or vanilla-extract
to name a few, you'll probably
notice that custom properties are not being resolved. This happens because each file is processed separately so
unless you import the custom properties definitions in each file, they won't be resolved.
To overcome this, we recommend using the PostCSS Global Data
plugin which allows you to pass a list of files that will be globally available. The plugin won't inject any extra code
in the output but will provide the context needed to resolve custom properties.
For it to run it needs to be placed before the PostCSS Custom Properties plugin.
const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');
const postcssGlobalData = require('@csstools/postcss-global-data');
postcss([
postcssGlobalData({
files: [
'path/to/your/custom-selectors.css'
]
}),
postcssCustomProperties()
]).process(YOUR_CSS );