A port of Simple Focus's FlowType.js without the jQuery dependency
npm install flowtype-js
const flowtype = require('flowtype-js');
flowtype(document.querySelector('#your-element'), {
minimum : 500,
maximum : 1200,
minFont : 12,
maxFont : 40,
fontRatio : 30
});
See simplefocus.com/flowtype/ for details on configuration options