Something I saw from the link someone provided to the thread, that seemed like a good point to bring up, is that any student using a screen reader, like someone visually impaired, might get caught up in that as well. Or for that matter, any student that happens to highlight the instructions, sees the hidden text, and doesnt realize why they are hidden and just thinks its some kind of mistake or something. Though I guess those students might appear slightly different if this person has no relevant papers to actually cite, and they go to the professor asking about it.