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 on each of the tabs, more CSS rules are applied and the percentage drops down to just ~15%.
That's why I'm so skeptical of anything that attempts to measure "unused CSS." This is an incredibly simple demo (all it does is click some tabs) and the amount of unused CSS changes dramatically.