var purifycssExtended = require("purifycss-extended")
var html = '<div class="span-2"></div>'
var css = `
/* only .span-2 should stay */
.span-1 {width: 1vw}
.span-2 {width: 2vw}
.span-3 {width: 3vw}
/* .extra will be removed */
.extra {width: 100vw}
`
var purified = purifycssExtended(html, css, {})
Created from: https://npm.runkit.com/purifycss-extended