I find M-q (fill-paragraph) is the command I miss the most using the built-in
text editor.

The value of `fill-column` will have to be different than it is in emacs, because
in a web browsing setting, I would think that the typical case would be to fill
to the width of the text box.  Perhaps we could designate the number 0 for that
purpose.

I could also see this as potentially useful for non-input cases. Specifically a long stream of text on a
webpage that isn't properly wrapped otherwise.

wrapping ordinary text in web pages would be more easily accomplished with a little DOM manipulation
and some CSS.