I’ve been doing it since ~2006. Contrast was not an issue back then, and my very young eyes wouldn’t have (and still don’t) cared.

I think maybe it was a manifestation of ADHD. Highlighting what I read helped me keep track of where I was if I got distracted (e.g. by a thought, which happens a lot). It also helps me if I get distracted or fail to parse a sentence - I can easily go back to the last known good state and restart reading.

Websites doing weird things on highlight (or preventing highlighting) piss me off to no end.

Another tick possibly due to ADD: I consistently close my browser tabs. I’ve met a lot of people who need special extensions so they can have 100+ tabs open, and that drives me absolutely nuts. If I’m done using it, I close it and if I need to come back later I google(DuckDuckGo really) it.