postcss uncss

index.js

const postcss = require('postcss')
const fs = require('fs')

const options = {
    html: ['html/*.html']
}

fs.readFile('css/index.css','utf8', (err, css) => {
  postcss([require('postcss-uncss')(options)])
    .process(css, { from: 'css/app.css', to: 'css/filtered.css' })
    .then(result => {
      fs.writeFile('css/filtered.css', result.css, () => true)
      if ( result.map ) {
        fs.writeFile('dest/app.css.map', result.map, () => true)
      }
    })
})