Earlier quoted context omitted.
A ridiculously cheap and powerful alternative to HTML (or any other static format for that matter), would be a bytecode format intended to be executed in a sandboxed virtual machine. Possibly something like Flash. That should give you all the bells and whistles you could possibly want with a document format. Now, there still are some loose ends about various screen sizes, preferred font family and size, and of course…
Really? Flash is more secure than HTML? Maybe we should just embed Java applets in emails? This still doesn't solve the "click a link that takes them elsewhere" issue that was brought up.
It would disable any form of cross scripting by default, which in my opinion is a better state of affairs than HTML alone is right now (now, you can embed an external image in a web page, and bam, Facebook knows what you read).