It could be a different format altogether - something third as the golden mean.
Alternatives: Markdown (we need specification for that) or HTML v2.0 or something else.
Requirements to such format:
- it shall provide only one way of text -> rendering and rendering -> text so WYSIWYG can be implemented in non-controversial manner.
If HTML then HTML v. 2.0 as the last version that supported WYSIWYG editing (as it has no CSS). CSS is allowed to be applied by email agent application (user's theming) but not from the body of the message.