The reason not to jazz up a signature is because it’s a signature. It’s not mail content, it’s the same in every mail you send. The interesting bit of the mail is what you want to send that’s unique to that mail, the content itself. So why draw the reader away from it with a 72pt flashing name with alien glow effect?

On the 8, 7, 5 line thing: netiquette is *3* lines, 72 characters max per line. Admittedly this dates back to 80×25 text screens, but if you’re putting in name/job title/phone numbers, why would you put one piece of information per line:

–
John Smith, Senior wombat fiddler, Acme Inc
Office: +1 234 567 8901 Cell: +1 234 567 8902

2 lines, all people want… You could add your preferred email address – it’s in the message headers already, but modern clients tend to ‘helpfully’ hide it from view, particularly when printed.

Generally: yes, HTML is supported, but there’s rarely much point to use it in emails. When I receive emails in bizarre nonstandard fonts, it looks like tarted up spam. I know what font I like to read my messages in – and when an email arrives in plaintext I get to choose. I might be poorly sighted or have a super high res monitor, and 12/10/9pt would then be far too small, for instance.