index.js

let postcss = require('postcss')

module.exports = postcss.plugin(
  'postcss-plugin-prefix',
  ({ prefix, matchRule }) => {
    // Work with options here

    return root => {
      root.walkRules(rule => {
        // don't touch @keyframes children
        if (rule.parent && rule.parent.name === 'keyframes') {
          return
        }
        rule.selectors = rule.selectors.map(selector => {
          // replace combinator selectors that can't be prefixed.
          selector = selector.replace(
            /^html\.body\.|^html\.|^body\./,
            prefix + '.',
          )

          // replace descendant combinators that can't be prefixed.
          selector = selector.replace(/^body$|^html$/, prefix)
          if (matchRule(selector)) {
            return `${prefix} ${selector}`
          }
          return selector
        })
      })
    }
  },
)