I started having this issue around 2020. I noticed the browser versions released late 2020 and onward all cause eye strain. I'm not sure why. I stopped being able to use Safari starting with version 14 on Mac OS X (ios is fine), Firefox started causing issues after version 89, and current versions of Chrome and Edge also are problematic. I speculate that something changed in the web standards support around that time where all of the browsers started rendering text/graphics in a new way. But I really have no idea.