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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.

Quick start

PostCSS Preset Env is a PostCSS plugin.
If you are already using PostCSS to build your CSS, you can simply add PostCSS Preset Env to your configuration.

Install postcss-preset-env from npm.
Add postcss-preset-env to your configuration.
Explore the features list.

const postcssPresetEnv = require('postcss-preset-env');

const yourConfig = {
	plugins: [
		/* other plugins */
		/* remove autoprefixer if you had it here, it's part of postcss-preset-env */
		postcssPresetEnv({
			/* pluginOptions */
			features: {},
		})
	]
}

How does it work?

PostCSS Preset Env is a Plugin Pack for PostCSS. It leverages the list of the features we keep an eye from CSSDB and applies plugins, so you can use those new features without having to worry about browser support.

CSSDB exposes the browser support that each feature has which can come from Can I Use or from MDN (through mdn/browser-compat-data).

By providing a list of browser targets for your project, plugins that aren't needed will be skipped. Over time your targets might change and by updating the settings your CSS bundle will only ever contain the needed fallbacks.

What PostCSS Preset Env does is to take the support data that comes from MDN and Can I Use and determine from a browserslist whether those transformations are needed. It also packs Autoprefixer within and shares the list with it, so prefixes are only applied when you're going to need them given your browser support list.

Glossary:

Browser list option: Browserslist is a package that gives you a list of browsers for a given query. For example, chrome < 42 will give you a list of every Chrome version that has been released up to, but not including, 42.
Browser support stats: Features get introduced on browsers at certain versions. They're often visible on MDN and Can I Use. Comparing these stats with the needed support for your project tells you if it's safe to use a feature or not.
CSS Feature: A CSS feature is often part of some spec that enables a specific feature. For example, hwb functional notation lets you express a given color according to its hue, whiteness, and blackness. This is part of the CSS Color 4 Spec.
CSS Spec: A Spec is a document that collects new features, what problems are they trying to solve and how it's intended to be solved (generally). This is usually an evolving document that goes over lengthy discussions between several people from different companies.
Plugin: A plugin is package that's intended (usually) to enable a new CSS Feature by leveraging PostCSS. This doesn't need to be part of any spec. An example of the latter is PostCSS Mixins, a concept that existed on Less or Sass, but it's not part of any spec. This plugin pack only packs plugins that enable features acknowledged by the World Wide Web Consortium (W3C) which will then be implemented by browsers later.
Polyfill: A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it. A polyfill should be indistinguishable from the native behaviour.

Here's a quick example of the syntax you can leverage by using PostCSS Preset Env.

@custom-media --viewport-medium (width <= 50rem);
@custom-selector :--heading h1, h2, h3, h4, h5, h6;

:root {
  --mainColor: #12345678;
}

body {
  color: var(--mainColor);
  font-family: system-ui;
  overflow-wrap: break-word;
}

:--heading {
  background-image: image-set(url(img/heading.png) 1x, url(img/heading@2x.png) 2x);

  @media (--viewport-medium) {
    margin-block: 0;
  }
}

a {
  color: rgb(0 0 100% / 90%);

  &:hover {
    color: rebeccapurple;
  }
}

/* becomes */

:root {
  --mainColor: rgba(18, 52, 86, 0.47059);
}

body {
  color: rgba(18, 52, 86, 0.47059);
  color: var(--mainColor);
  font-family: system-ui, -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Oxygen, Ubuntu, Cantarell, Droid Sans, Helvetica Neue;
  word-wrap: break-word;
}

h1, h2, h3, h4, h5, h6 {
  background-image: url(img/heading.png);
}

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  h1, h2, h3, h4, h5, h6 {
    background-image: url(img/heading@2x.png)
  }
}

@media (max-width: 50rem) {
  h1, h2, h3, h4, h5, h6 {
    margin-top: 0;
    margin-bottom: 0;
  }
}

a {
  color: rgba(0, 0, 255, 0.9)
}

a:hover {
  color: #639;
}

Without any configuration options, PostCSS Preset Env enables Stage 2 features and supports all browsers.

Try it out in the Playground!

Note

Please note that some features need a companion library that makes the feature work. While we try to avoid this requirement, there are instances in which this isn't possible to polyfill a new behaviour with just CSS.

See the list below.

Usage

Add PostCSS Preset Env to your project:

npm install postcss-preset-env --save-dev

Use PostCSS Preset Env as a PostCSS plugin:

const postcss = require('postcss');
const postcssPresetEnv = require('postcss-preset-env');

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

PostCSS Preset Env runs in all Node environments, with special instructions for:

Position in your PostCSS plugins list

PostCSS Preset Env only contains polyfills and fallbacks for standard CSS features and is unable to handle non-standard features and syntactic sugar.

If you also have PostCSS plugins for non-standard features and syntactic sugar,
you should place these first in the PostCSS plugins list.

module.exports = {
	plugins: [
		"postcss-syntactic-sugar",
		"postcss-non-standard",
		// ...
		[
			"postcss-preset-env",
			{
				// plugin options
			},
		],
		// ...
		// maybe a minifier?
	],
};

You can also use the insertBefore / insertAfter plugin options for more fine grained control.

Options

stage

The stage option determines which CSS features to polyfill, based upon their stability in the process of becoming implemented web standards.

postcssPresetEnv({ stage: 0 })

The stage can be 0 (experimental) through 4 (stable), or false. Setting stage to false will disable every polyfill. Doing this would only be useful if you intended to exclusively use the features option.

Default: 2

It is very rare for features to progress beyond stage 2.
Use minimumVendorImplementations if you prefer to keep up with modern features.

minimumVendorImplementations

The minimumVendorImplementations option determines which CSS features to polyfill, based their implementation status. This can be used to enable plugins that are available in browsers regardless of the spec status.

postcssPresetEnv({ minimumVendorImplementations: 2 })

minimumVendorImplementations can be 0 (no vendor has implemented it) through 3 (all major vendors).

Default: 0

Note:

When a feature has not yet been implemented by any vendor it can be considered experimental.
Even with a single implementation it might still change in the future.
Sometimes issues with a feature/specification are only discovered after it becomes available.

A value of 2 is recommended when you want to use only those features that should be stable.

Having 2 independent implementations is a critical step in proposals becoming standards and a good indicator of a feature's stability.

features

The features option enables or disables specific polyfills by ID. Passing true to a specific feature ID will enable its polyfill, while passing false will disable it. List of Features

postcssPresetEnv({
  /* use stage 3 features + css nesting rules */
  stage: 3,
  features: {
    'nesting-rules': true
  }
})

Passing an object to a specific feature ID will both enable and configure it.

postcssPresetEnv({
  /* use stage 3 features + custom-selectors (preserving the original CSS) */
  stage: 3,
  features: {
    'custom-selectors': { preserve: true }
  }
})

If you want to preserve automatic enabling of features
based on the stage, implementations and or browserslist,
you can pass an array: ['auto', { /* plugin options */ }].

postcssPresetEnv({
  /* use stage 3 features + custom-selectors (preserving the original CSS) */
  stage: 3,
  features: {
    'custom-selectors': ['auto', { preserve: true }]
  }
})

Any polyfills not explicitly enabled or disabled through features are determined by the stage option.

env

PostCSS Preset Env supports standard browserslist configuration, which can be a .browserslistrc file, a browserslist key in package.json, or browserslist environment variables.

The env option is used by browserslist to determine the named environment that should be used when you have multiple browserslist environments configured. If not set, Browserslist will use the production environment.

/* use the environment named `development`, instead of the default environment of `production` */
postcssPresetEnv({ env: 'development' })

browsers

The browsers option overrides any existing browserslist configuration.

The browsers option should only be used when a standard browserslist configuration is not available.
When the browsers option is used the env option will be ignored.

postcssPresetEnv({ browsers: 'last 2 versions' })

If no valid browserslist configuration is specified, the default browserslist query will be used.

insertBefore / insertAfter

The insertBefore and insertAfter keys allow you to insert other PostCSS plugins into the chain. This is only useful if you are also using sugary PostCSS plugins that must execute before or after certain polyfills. Both insertBefore and insertAfter support chaining one or multiple plugins.

import postcssSimpleVars from 'postcss-simple-vars';

postcssPresetEnv({
  insertBefore: {
    'all-property': postcssSimpleVars
  }
})

autoprefixer

PostCSS Preset Env includes autoprefixer; the env and browsers options will be passed to it automatically.

Specifying the autoprefixer option enables passing additional options into autoprefixer.

postcssPresetEnv({
  autoprefixer: { grid: true }
})

Passing autoprefixer: false disables autoprefixer.

⚠️ autoprefixer has complex logic to fix CSS Grid in IE and older Edge.

This can have unexpected results with certain features and when preserve: true is used. (defaults to true)

:root {
  --grid-gap: 15px;
}

.test-grid {
	grid-gap: var(--grid-gap);
	grid-template-columns: repeat(2, 1fr);
}

Becomes :

.test-grid {
	grid-gap: 15px;
	grid-gap: var(--grid-gap);
	-ms-grid-columns: 1fr var(--grid-gap) 1fr;
	grid-template-columns: repeat(2, 1fr);
}

The prefixed -ms-grid-columns still has a custom property: 1fr var(--grid-gap) 1fr; which won't work.
This example shows issues with custom properties but other transforms might have similar issues.

If you target IE or older Edge it is best to avoid using other modern features in grid property values.
As a last resort you can set preserve: false, we do not advice this as doing so purely to fix issues with CSS grid.

older Edge is any version before chromium (<79)

preserve

The preserve option determines whether all plugins should receive a preserve option, which may preserve or remove otherwise-polyfilled CSS. By default, this option is not configured.

postcssPresetEnv({
  preserve: false // instruct all plugins to omit pre-polyfilled CSS
});

debug

The debug option enables debugging messages to stdout which should be useful to help you debug which features have been enabled/disabled and why.

enableClientSidePolyfills

The enableClientSidePolyfills enables all features that also need an extra browser library to be loaded into the page for it to work. Defaults to false.

Note that manually enabling/disabling features via the "feature" option overrides this flag.
This only controls if the PostCSS plugins are enabled. It does not cause the browsers libraries to be included in your bundle.

logical

The logical option can hold an object which lets you specify direction of the inline and block axes and will affect the following features:

logical-properties-and-values: PostCSS Logical
float-clear-logical-values: PostCSS Logical Float And Clear
logical-resize: PostCSS Logical Resize
logical-viewport-units: PostCSS Logical Viewport Units

It should have blockDirection and/or inlineDirection which can be any of the following:

top-to-bottom
bottom-to-top
left-to-right
right-to-left

postcssPresetEnv({
  logical: { // instruct all logical plugins to set inline axis to right to left
		inlineDirection: 'right-to-left',
	},
});

.element {
	float: inline-start;
	padding-inline-end: 10px;
}

Becomes :

.element {
	float: right;
	padding-left: 10px;
}

You can't mix two vertical directions or two horizontal directions so for example top-to-bottom and right-to-left are valid, but top-to-bottom and bottom-to-top are not.

You might want to tweak these values if you are using a different writing system, such as Arabic, Hebrew or Chinese for example.

Stability and Portability

PostCSS Preset Env will often include very modern CSS features that are not fully ready yet. This gives users the chance to play around with these features and provide feedback.

If the specification changes or is abandoned a new major version of the plugin will be released. This will require you to update your source code so that everything works as expected.

To have more stability between updates of PostCSS Preset Env you may set stage: 3 and/or minimumVendorImplementations: 2.

A side effect of staying close to the standard is that you can more easily migrate your project to other tooling all together.

Plugins list

Plugins that need client library

This is the current list of features that need a client library with a link to the polyfill's library.

blank-pseudo-class: Plugin / Polyfill
focus-visible-pseudo-class: Plugin / Polyfill
focus-within-pseudo-class: Plugin / Polyfill
has-pseudo-class: Plugin / Polyfill
prefers-color-scheme-query: Plugin / Polyfill

If you want to enable all these types of features, please check the enableClientSidePolyfills option.

Plugins not affected by Browser Support

Given they have no support they will always be enabled if they match by Stage:

blank-pseudo-class: Plugin / Polyfill
custom-media-queries: Plugin
nesting-rules: Plugin

Uh oh!