While email content is in fact of great importance, my pet peeve is the catastrophe that is the HTML used for such emails.

I mean, it's even uglier than the competing HTML used in the 90s, and that's saying something.

How do I prove this? Easy, we used to have browser wars between at most 3. Well, currently we have email wars* between at least 6 different (major) mail clients.

* ok, not really a war, since they don't seem to care as much...