What guidelines can you give for rich HTML formatting in emails while maintaining good visual stability across many clients and web based email interfaces?

An unrelated answer on a question on Stack Overflow suggested:

http://www.campaignmonitor.com/blog/archives/2008/05/2008_email_design_guidelines.html

Which contains the following guidelines:

  1. Place stylesheet in <body> instead of <head>
    Some email clients will strip CSS out of the head, but leave it if the style block is (invalidly) in the body.
  2. Use inline styles where ever possible
    Gmail will strip any stylesheet, whether in the <head> or in the <body>, but honor inline styles assigned using the style="" attribute
  3. Return to tables
    Email standards have actually taken a giant step backwards in recent years thanks to Outlook 2007 using the Microsoft Word rendering engine. Unlearn most of what you learned about positioning without stylesheets.
  4. Don't rely on images
    Most clients and most web based email clients will not display images unless the user specifically requests them to be displayed.

I also have a few "unconfirmed" truths that I don't remember where I read them.

  1. Don't use more than two levels of nesting in tables
    Is this true. What is likely to happen if I do? Is there any particular client/clients that choke on this?
  2. Be careful of nesting background images in cells/tables
    As I understand you may encounter situations where the background image is applied in the descending table/cell completely anew, and not just "shining through". Again, true or not? Which clients?
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