Of course if you're using bayesian filtering then the mispellings won't matter as they will automatically migrate to a high spam probability (e.g. the filters I have here show spellings such as offerz, ratez right alongside HTML tags as major spam indicators)

It's exactly that sort of abuse (links back to sites) which is why spammers love HTML email and probably why HTML in email is one of the biggest indicators of spam (looking at an analysis of my own company's email HTML formatting is in fact a more reliable indicator of spam than sexually related terms in the content!).

But if all they send is HTML then the recipient may well see nothing if they strip unsafe emails or gibberish if they don't want to open any HTML. Sending multipart email so there is a clear text safe copy is a step in the right direction but still leaves the recipient downloading huge chunks of rubbish that isn't required. Formatting for emphasis is safe only as long as you know the exact situation where a document will be read. You may well want to emphasise this sentence by making it red with parts in italic. What the recipient may see is. 'You may well want to emphasise this sentence by making it red with parts in italic'. If they quote it the emphasis is lost and the entire emphasised section may be invisible on a printed copy. Just a couple of reasons why HTML email is a bad idea.

HTML email! If you need pretty pictures, colours and fonts to say what you want to say then you should be using different words not different formatting.