I would be interested in reading more about how you prevent web designers from changing the page's font. I used about:config on Firefox to stop fonts from downloading, but that made it hard for me to use github.com and one or 2 other sites because those sites use a font to render icons that are essential parts of the UI.
In Firefox: Options – Language and Appearance – Fonts and Colors – Advanced… – Disable “Allow pages to choose their own fonts, instead of your selections above”
It's what I've been using for months now (and I also block web fonts using Firefox Focus' extension in iOS/iPadOS Safari). And that's even though I'm a typography geek, and have bought fonts from Matthew Butterick (the author of the submitted article).
Looks weird for a day or two, after that you never miss web fonts.
Make sure to select a nice default font (for example, Verdana is nice, but I'm German, and Verdana does not have a closing quotation mark for German – it's another one than is in use in English).
My setting is Constantia as default font and serif, Segoe UI as sans-serif, and Consolas as monospace.
