A more reliable approach, though requiring a reload, is to block web fonts in uBlock Origin (which you’re using, right?)—there’s a button for the purpose.<p>You can also just turn font choice off altogether, so that you just get your chosen serif/sans-serif/monospace/whatever fonts. In Firefox, it’s Settings → Fonts → Advanced → untick <i>Allow pages to choose their own fonts, instead of your selections above</i>. I started doing that a couple of years ago as an experiment and have never gone back: it makes the web <i>so</i> much better with minimal and typically unimportant breakage.
> When the stylesheets are not accessible by document.styleSheets API (e.g. on a third party domain, what!?)<p>I agree with the !? sentiment. An even bigger WTF, though, is that the server gets to control through CSP which off-site style sheets the user can link against and even whether use of the style attribute is permitted. CSP is one of the worst pieces of Web security infrastructure that modern browsers unwisely agreed to go along with. Incredibly user-hostile and worse, effectively irreversible—there's basically no going back now because browsers don't have the option at this point to say, "Yeah, we're not going to honor this anymore," without putting users at an even <i>greater</i> risk than before CSP ever existed. So not just detestable in the vein of WEI, but an utterly abominable technical work.
I might sound like a boomer, but I still use some...<p>Simple dark mode <a href="https://pastebin.com/Dfxtd6hJ" rel="nofollow">https://pastebin.com/Dfxtd6hJ</a><p>This one for responsive previews <a href="http://lab.maltewassermann.com/viewport-resizer/" rel="nofollow">http://lab.maltewassermann.com/viewport-resizer/</a>