My web browser extension StopTheMadness Pro has a font replacement feature that allows you to replace the fonts chosen by the website with fonts chosen by you. This feature is implemented by automatically adding CSS @font-face rules to the page style. Unfortunately, @font-face rules don't work to override the default system font, San Francisco on Apple devices, specified in CSS by a font-family declaration such as -apple-system, system-ui, or ui-sans-serif. (Don't ask me why this doesn't work...
