Awful superhero TV show? FUCK YOU, that was the best super hero tv show ever! THE GREATEST AMERICAN HERO!
Anyway, to answer your question, I couldn't find any option to leave html on for sigs only, so that's why it's off. Everything you really need for a sig is there anyway (minus image resizing, which I prefer to have disabled anyway to decrease the amount of people that use huge pics that take long to load in their sigs)