Critical Css makes your site render blazingly fast — especially on mobile. Implementing critical css will improve your performance score on tools like WebPageTest and Google’s Pagespeed Insights, and improve your SEO.
CSS Stylesheets block rendering. Until a user’s browser has requested, received, downloaded and parsed your stylesheets, your page will remain blank. By reducing the amount of CSS the browser has to go through, we can get the page to render much, much faster.
Unfortunately it’s quite tedious to manually determine what css should be considered critical for a page. This tool removes that pain by automating this process.