RunKit + npm: purgecss

const Purgecss = require('purgecss'); const fs = require('fs'); const css = '.a{color:#fff}.b{color:#000}.c{color:#123}'; fs.writeFileSync('a.css', css, err => console.log.bind(console)); const content = '<b class="a"></b>'; fs.writeFileSync('b.html', content, err => console.log.bind(console)); new Purgecss({ content: ['b.html'], css: ['a.css'] }).purge();