Icon fonts that use the Private Use Area code points do still work when you just instruct the browser to not let pages override your choices. This is the way almost all icon fonts work. In the last few months I think I’ve observed only three fonts not doing so; DuckDuckGo is one, the second was some small business’s site using a style from many years ago, and the third is Google’s Material Icons font, distressingly widely used, which uses a ligation technique the implications of which really weren’t thought through properly. (It was supposed to improve accessibility in case of the font not loading, but in practice it makes it disastrously bad much more often, as can be seen on a number of Google properties, like their docs sites and Google Translate which are both significantly mangled by it.)