I'm getting the info from testing. I tried using Yesod for a while but kept getting this error:

    "A tag name may not contain a slash. Perhaps you have a closing tag in your HTML."

Which you can find in the source for the parser as well:

    if '/' `elem` tn
          then fail "A tag name may not contain a slash. Perhaps you have a closing tag in your HTML."

Maybe there's a way to work around this so you can use closed tags, but it sure wasn't obvious.