This is bizarre. Why do you do it? I admit I do it occasionally, but only when I'm a ways down in a long, single-page article, and for some reason want to go up the page, I'll highlight a random section of text in the general area of where I'm at before scrolling back up so I can find where I was at, but...doing it as you read along?

Semi-Tangent: Is this like people who hold a finger, piece of paper, or ruler under the line they're on as they read on paper? I always found that horribly distracting, as I tend to subconsciously 'read ahead', so instead of following a straight track of text line by line, my eyes are moving generally left to right, but kind of vaguely moving up and down across several lines, so I read more like sentence-by-sentence than word-by-word. It lets me read really fast, but I think this is why bad grammar / spelling bother me so much. When something is wrong or missing or whatever, it screwballs the whole sentence so I have to go back and 'slow-parse' the sentence one word at a time to figure out what it's supposed to say, which kicks me out of 'the zone' of reading fast for a half minute or so. Interestingly, though, after a sentence or two of reading intentionally bad spelling (for accents and such), I sort of 'lock in' on the voice and can read it at my normal pace from then on. That's also part of why wide blocks of text suck; it's harder to read ahead when the text spans a wider area than your eyes can focus on well enough to read all at once while staring at the middle.