PostCSS Critical CSS

This plugin allows the user to define and output critical CSS using custom atRules, and/or custom CSS properties. Critical CSS may be output to one or more files, as defined within the plugin options and/or within the CSS. Depending on the plugin options used, processed CSS may be left unchanged, or critical CSS may be removed from it.

Install

npm install @veritashealth/postcss-critical-css --save

Examples

An example is available in this repo. See the /example directory, and use the command npm run example to test it out.

Usage examples

All examples given below show the input CSS and the critical CSS that is output from it. Note that the input CSS will remain unchanged, unless preserve is set to false in the plugin options. Use npm run example to see how this works.

Using the @critical atRule

/* In foo.css */
@critical;

.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Will output:

/* In critical.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Using the @critical atRule with a custom file path

/* In foo.css */
@critical bar.css;

.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Will output:

/* In bar.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Using the @critical atRule with multiple custom file paths

/* In foo.css */
@critical bar.css baz.css;

.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Will output:

/* In bar.css and also in baz.css*/
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Using the @critical atRule with a subset of styles

/* In foo.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

@critical {
  .bar {
    border: 10px solid gold;
    color: gold;
  }
}

Will output:

/* In critical.css */
.bar {
  border: 10px solid gold;
  color: gold;
}

Using the @critical atRule with a subset of styles

/* In foo.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

@critical critical-1.css critical-2.css {
  .bar {
    border: 10px solid gold;
    color: gold;
  }
}

Will output:

/* In critical-1.css and also in critical-2.css */
.bar {
  border: 10px solid gold;
  color: gold;
}

Using the custom property, critical-selector

/* In foo.css */
.foo {
  critical-selector: this;
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Will output:

/* In critical.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Using the custom property, critical-selector, with a custom selector.

/* In foo.css */
.foo {
  critical-selector: .bar;
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Will output:

/* In critical.css */
.bar {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Using the custom property, critical-filename

/* in foo.css */
.foo {
  critical-selector: this;
  critical-filename: secondary-critical.css;
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Will output:

/* In secondary-critical.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

Using the custom property, critical-selector, with value scope

This allows the user to output the entire scope of a module, including children.

/* in foo.css */
.foo {
  critical-selector: scope;
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

.foo a {
  color: blue;
  text-decoration: none;
}

Will output:

/* In critical.css */
.foo {
  border: 3px solid gray;
  display: flex;
  padding: 1em;
}

.foo a {
  color: blue;
  text-decoration: none;
}

Plugin options

The plugin takes a single object as its only parameter. The following properties are valid:

| Arg | Type | Description | Default | ---|---|---|---| | outputPath | string | Path to which critical CSS should be output | Current working directory | | outputDest | string | Default critical CSS file name | "critical.css" | | preserve | boolean | Whether or not to remove selectors from primary CSS document once they’ve been marked as critical. This should prevent duplication of selectors across critical and non-critical CSS. | true | | minify | boolean | Minify output CSS? | true | | destDelim | string | Delimiter that separates multiple critical destination files. | ” “ |