It's pretty obvious that underlining is something that rarely appears in print. It is mainly an annotation device: something added to finished print, by hand.
When was the last time you saw underlines in a book that weren't added by a previous reader?
Underlines are bad typography that should be used sparingly, if ever.
It's even more obvious that we're not talking about print, but a markup language anyone can outside of book publishing. So why do you want to prevent a "previous reader" from using said annotation device for the little note he's creating/editing?
We got here because you mentioned all the irrelevant stuff your learned in high school, and when I pointed that out, you mentioned some more, now about the whole underline, which I used as an example of how there is no intuition behind _ since that intuition is "taken" by underlines (that's why you needed to criticize underlines)
