Do you intend to offer the HTML option of underline for comments anytime in the near future? I appreciate your full line of existing HTML commands, but I'm surprised that underline is not an available command.
The general rule is that you italicize what can stand on its own, ie a novel/book, long poem, etc. You use quotation marks/underline smaller works that don't stand on their own, aka are a part of a larger worker, like chapters, short poems in a book of poems, or an article.
I find that a surprising request. Bold is for emphasis, and italics for titles. Why would you need underline when can (and perhaps should) italicize instead?
I'm wary of adding underline for the same reason you don't see it used much on the web; underlines are reserved for links. It's confusing to have underlined text that's not a link.
I personally use underline when I want someone to notice something but it's not massive enough to use bold. I also discuss books and need underline to show the titles.
There are a myriad of uses for underline; adhering to proper MLA standards is just one of them. I don't think I am alone by any means in yearning for the ability to underline text. For me, the most important use it would serve is to highlight text within a quote without changing the original font. For example, I like to post articles with figures and data that I want to make explicitly clear to my audience, yet I don't like to use bold, because as I mentioned, it distorts the original font of the text.
Once again, there are many uses for underlining text, and Fae and ...
Also, modern MLA standards now call for italics instead of underlining. Look it up. Underlining is essentially outdated.