Never. Never never never. When sites do this, the only words my eyes can read are the ones with the background shifted on them. Since we added this, I can’t read MDN at all. I’ve never understood why anyone finds this readable or helpful in any way.
This isn’t even hyperbole. I outright can’t read MDN now that we’ve done this. My eyes are not able to focus on the words that aren’t highlighted (and yes, even though it’s a light gray, it’s still a highlight to my eyes and brain). This is why I always bought new textbooks in college – to be sure there were no highlighted bits that would make pages unreadable. Cost a fortune but saved my grades, most likely.
It makes work very challenging when I can’t read the content I’m supposed to be working on updating…