You can only use HTML tags in the body
of the message. Your html markup will appear in the posting.

Why are there colons in the message
when I try to post a followup?

Colons appear in the message dialog box when you try to follow up
on a message to indicate that those lines are quoting
the previous document.

Why didn't my post show up?

Your post most likely did not show up, because your browser did not reload the page, it simply pulled it out of cache. Please reload your browser and it should then appear.