From Johnny’s dev blog:
The challenge: Calculate the real percentage of unused CSS
Our goal is to create a script that will measure the percentage of unused CSS of this page. Notice that the user can interact with the page and navigate using the different tabs.
DevTools can be used to measure the amount of unused CSS in the page using the Coverage tab. Notice that the percentage of unused CSS after the page loads is ~55%, but after clicking …
Monitoring unused CSS by unleashing the raw power of the DevTools Protocol is a post from CSS-Tricks