PostCSS :dir() lets you use the :dir pseudo-class in CSS.

.example:dir(rtl) {
  margin-right: 10px;
}

.example:dir(ltr) {
  margin-left: 10px;
}

/* becomes */

[dir="rtl"] .example {
  margin-right: 10px;
}

[dir="ltr"] .example {
  margin-left: 10px;
}

If your browserslist already supports the :dir pseudo-class, this plugin will not change your CSS. You can learn more this by reading about the browsers option.

PostCSS :dir() does not change selector weight, but does require at least one [dir] attribute in your HTML. If you don’t have any [dir] attributes, consider using the following JavaScript:

// force at least one dir attribute (this can run at any time)
document.documentElement.dir=document.documentElement.dir||'ltr';

If you absolutely cannot add a [dir] attribute in your HTML or force one via JavaScript, you can still get around this by presuming a direction in your CSS using the dir option, but know that this will increase selector weight by one element (html).

Add PostCSS :dir() to your build tool:

npm install postcss-dir-pseudo-class --save-dev

Use PostCSS :dir() to process your CSS:

require('postcss-dir-pseudo-class').process(YOUR_CSS);

Add PostCSS to your build tool:

npm install postcss --save-dev

Use PostCSS :dir() as a plugin:

postcss([
  require('postcss-dir-pseudo-class')()
]).process(YOUR_CSS);

Add Gulp PostCSS to your build tool:

npm install gulp-postcss --save-dev

Use PostCSS :dir() in your Gulpfile:

var postcss = require('gulp-postcss');

gulp.task('css', function () {
  return gulp.src('./src/*.css').pipe(
    postcss([
      require('postcss-dir-pseudo-class')()
    ])
  ).pipe(
    gulp.dest('.')
  );
});

Add Grunt PostCSS to your build tool:

npm install grunt-postcss --save-dev

Use PostCSS :dir() in your Gruntfile:

grunt.loadNpmTasks('grunt-postcss');

grunt.initConfig({
  postcss: {
    options: {
      use: [
        require('postcss-dir-pseudo-class')()
      ]
    },
    dist: {
      src: '*.css'
    }
  }
});

If your browserslist already supports the :dir pseudo-class, this plugin will not change your CSS. While only Firefox currently supports :dir, this will surely improve over time.

Here’s an example of a package.json using a browserslist that would fully support the :dir pseudo-class:

{
  "browserslist": "firefox >= 49"
}

And here’s an example of using the browsers option to accomplish the same thing:

require('postcss-dir-pseudo-class')({
  browsers: 'firefox >= 49'
});

In both of these examples, the CSS would remain unchanged.

.example:dir(rtl) {
  margin-right: 10px;
}

/* becomes */

.example:dir(rtl) {
  margin-right: 10px;
}

By default, this plugin requires you to specify a direction [dir] attribute in your HTML, preferably on the html element. If you prefer not to, you can presume a direction in your CSS using the dir option.

Here’s an example of using the dir option to presume a left-to-right direction:

require('postcss-dir-pseudo-class')({
  dir: 'ltr'
});

.example:dir(ltr) {
  margin-left: 10px;
}

.example:dir(rtl) {
  margin-right: 10px;
}

/* becomes */

:not([dir="rtl"]) .example {
  margin-left: 10px;
}

[dir="rtl"] .example {
  margin-right: 10px;
}

Note: The :root pseudo-class is added here to preserve the weight of the original selector.