How to enable StopTheFonts:
Purchase and install StopTheFonts in the App Store.
After you install StopTheFonts on your Mac, you should see "New Extension Available" in Safari.
If you don't see "New Extension Available", then just open the Extensions tab of the Preferences window in Safari. The "Enable in Safari…" button in the StopTheFonts app opens this automatically.
In the Extensions tab of the Safari Preferences window, check the box next to StopTheFonts to enable it.
Make sure that Content Blockers are On in the Websites tab of the Safari Extensions window.
How to use StopTheFonts:
StopTheFonts has three block modes: None, 3rd Party, or All. StopTheFonts doesn't block any web fonts by default. In block All mode, StopTheFonts blocks all web fonts by default. In block 3rd Party mode, StopTheMadness blocks only 3rd party web fonts by default.
A 3rd party font is loaded from a different domain from the web page (for example, https://fonts.gstatic.com vs. https://underpassapp.com), whereas a 1st party font is loaded from the same domain as the web page. You can look at a test page with examples of 1st party and 3rd party web fonts.
You can create special rules to handle specific URLs. The plus button adds a rule, and the minus button removes a rule. There are two kinds of URL governed by the rules: web font URLs and web site URLs. Web font URL rules block or allow specific web fonts on every web site. Web site URL rules, on the other hand, block or allow web fonts on specific web sites, regardless of the URL of the font.
Web site URL rules take precedence over web font URL rules, so if you block or allow web fonts on a particular site, StopTheMadness will ignore any web font URL rules on that site.
Safari content blocker rules require that all URLs be lowercase and ASCII. If you need non-ASCII characters in a URL, you can use punycode.
Safari content blocker rules allow a limited form of regular expressions. Check the Regex box if your URL uses regex. Some advanced regex features may not work in Safari content blocker rules, however, so make sure to test whether your regex works right.