Stoyan is absolutely correct. As much as we all love CSS, it’s still an important player in how websites load and using less of it is a good thing. He has a neat new bookmarklet called CSS Me Not to help diagnose unnecessary CSS files, but we’ll get to that in a moment.
The [problem] is that CSS is in the critical path, it blocks rendering and often even JavaScript execution. We love CSS, it’s magic, it can do unbelievable feats and fix broken UIs and manipulate images and draw amazing pictures. We love CSS. We just want… less of it, because of its inherently blocking nature.
Sometimes our sites use entire stylesheets that are simply unnecessary. I hate to admit it, but WordPress is a notorious offender here, loading stylesheets for plugins and blocks that you might not even really be using. I’m in that position on this site as I write. I just haven’t found the time to root out a couple of little stylesheets I don’t need from loading.
Stoyan created a quick bookmarklet called CSS Me Not to see all those CSS files. The big benefit, of course, is that it lets you know what you’re up against.
You could find these stylesheets in DevTools as well, but the CSS Me Not bookmarklet makes it extra easy and has a killer bonus feature: turning off those stylesheets. Testing the bookmarklet here on CSS-Tricks, I can see four stylesheets that WordPress loads (because of settings and plugins) that I know I don’t need.
If you wanted to do this in DevTools instead, you could filter your Network requests by CSS, find the stylesheet that you want to turn off, right-click and block it, and re-load.
I’ve been fighting this fight for ages, dequeuing scripts and styles in WordPress that I don’t want.
Removing totally unused stylesheets is an obvious win, but there is the more squirrely issue of removing unused CSS. I mention in that post the one-true-way of really knowing if any particular CSS is unused, which is attaching a background-image to every selector and then checking the server logs after a decent amount of production time to see which of those images were never requested. Stoyan corroborates my story here:
UnCSS is sort of a “lab”. The “real world” may surprise you. So a trick we did at SomeCompany Inc. was to instrument all the CSS declarations at build time, where each selector gets a 1×1 transparent background image. Then rummage through the server logs after a week or so to see what is actually used.