The simple Critical CSS generator is based on a browser-trick that was originally published by Google engineer Paul Kinlan. The snippet uses a Chrome innovation called
getMatchedCSSRules that was deprecated and has been removed since Chrome 63.
The simple Critical CSS generator uses an enhanced version of the snippet and supports modern browsers via a polyfill. An added feature is the ability to set the viewport to support responsive designs.
The quality of the simple critical CSS generator is often not that well but it can provide a good start base for designers that want to manually write pixel perfect Critical CSS.