You can do this in the browser already. In Firefox, go to Preferences → Language and Appearance → Advanced and untick "Allow pages to choose their own fonts, instead of your selections above".

The user experience is that it improves the vast majority of sites, by not having to read in 80 different fonts on 100 different sites, and breaks a few sites in minor ways because some text ends up overflowing their boundaries.

It would be great if this became the norm again. Content is king!