docusaurus/packages/docusaurus-cssnano-preset/src/remove-overridden-custom-properties/index.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

/**
 * This PostCSS plugin will remove duplicate/same custom properties (which are actually overridden ones) **only** from `:root` selector.
 *
 * Depending on the presence of an `!important` rule in value of custom property, the following actions will happens:
 *
 * - If the same custom properties do **not** have an `!important` rule, then all of them will be removed except for the last one (which will actually be applied).
 * - If the same custom properties have at least one `!important` rule, then only those properties that do not have this rule will be removed.
 * @returns {import('postcss').Plugin}
 */
module.exports = function creator() {
  return {
    postcssPlugin: 'postcss-remove-overridden-custom-properties',
    Declaration(decl) {
      if (decl.parent.selector !== ':root') {
        return;
      }

      const sameProperties = decl.parent.nodes.filter(
        (n) => n.prop === decl.prop,
      );
      const hasImportantProperties = sameProperties.some((p) =>
        Object.prototype.hasOwnProperty.call(p, 'important'),
      );

      const overriddenProperties = hasImportantProperties
        ? sameProperties.filter(
            (p) => !Object.prototype.hasOwnProperty.call(p, 'important'),
          )
        : sameProperties.slice(0, -1);

      overriddenProperties.map((p) => p.remove());
    },
  };
};

module.exports.postcss = true;