When I was doing my SqualorID, I wanted to make it look nicer than just some text and a gif, so I clicked the handy-dandy HTML button - and immediately butted against the restrictions.
It raised the question for me - what does work in the HTML section? I managed to get an image to float/align left, but trying to reduce the massive margin/padding on the side of it seemed to be impossible. I tried to add a background in my text area and failed at that as well, then I decided that the HTML tag is just taunting me and gave up on it.
Is there a way to know what you can and can't do in HTML without just trying it, saving, and tearing your hair out in frustration when it doesn't work?