The monospaced or proportional fonts aren't the issue; it's that HTML treats all sequences of whitespace within text as a single space. It's quite annoying to those of us who like our double-spaced sentences. When a word processor generates HTML, every double-spaced sentence ends with this:
HTML and proportional fonts are two separate issues. HTML ignores them, which is one problem; two spaces in a proportional font being less immediately obvious than on a typewriter is another problem. You can come up with some other issues too if you think about it. It all adds up to a dead tradition.
