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An email signature can be as simple as a few lines of code with a jpeg image email signature (examples) hosted on your webserver, to actual unicode ascii selectable text email signatures formatted for cross platform portability.
An HTML email signature, which is using background patterns and actual typed text (as opposed to the jpeg), and sometimes, more work and effort is put into crossbrowser testing in AOL (multiple versions), Comcast, Earthlink, Gmail, Hotmail, Lotus Notes, Mail.com, Outlook (multiple versions), Thunderbird, Windows Live, Yahoo!, Mail.app. Not to mention mobile clients. Because of certain empirical conflicts in the code restrictions on all these clients, I balance out the perfect signature, with minimal code as a strong template. Troubleshoot, test, put the styles inline, etc.