I don’t even highlight as I read, but I would close such a page out of principle.
(I am sure the people who do it consider those who don't weird!)
I think it started in like 1998 when I was a kid using ancient slow browsers on tiny fuzzy monitors over dialup speed internet. I'd be in the middle of reading an article/forum/etc and something would make me lose my place, like a picture (finally) loading above, or trying to scroll at 7 fps and the page jumping way past where I was (I never trusted pgdn), and it would be quite the chore to find my place again. So I started selecting around where I was reading, especially before scrolling, just so I can easily find my place again and it just became a habit.
Now I'm using a giant WQHD monitor at 144 Hz using the fastest page renderer in the world (webrender), so I really don't have those excuses anymore, yet the habit remains. At this point it's probably more ADD and habit than anything else; if I'm not selecting text, I'm fidgeting with something, a pen, usb drive, etc.
Highlighting text as I read helps me out when I'm having trouble focusing on the page.
Yeah, it's a foreach loop. I remember my keywords.
# Set x to 1
x = 1
...not even kidding.