npm i --save-dev purgecss-with-wordpress

import Purgecss from 'purgecss'
import purgecssWordpress from 'purgecss-with-wordpress'
​
const purgeCss = new Purgecss({
  content: ['**/*.html'],
  css: ['**/*.css'],
  whitelist: purgecssWordpress.whitelist,
  whitelistPatterns: purgecssWordpress.whitelistPatterns
})
const result = purgecss.purge()