I doubt most administrators want users posting items in html; all it takes is one nimrod to ruin a site (such as posting a javascript which would make every image load as a photo of a dancing bananna for example).
I think that html signatures can be entered via the cp, but I'm unsure as to how things parse out (as I've never toyed with html in signatures); I don't even know if theres an option to allow html emails in the current build (as again I've not played with it)