purify-css-inliner

to install:

npm install purify-css-inliner

using the library:

via command line:

purifycssinliner <filename>.html

can also be used as a package:

let purify = require('purify-css'),
  inliner = require('../src/purifycssinliner')(purify)
inliner(process.argv[2], (output) => {})

what it do:

this gathers css from link tags on an HTML file, concatenates them together, and passes everything to the purify-css library. then the output is inserted into the page as critical css