If You Say You’re Gonna Play Nice, Play Nice!
Outlook is broken, let’s fix it.
There is no widely-recognized consensus in the industry about what subset of HTML is appropriate for use in e-mail for interoperability. The “Email Standards Project” does not represent a sanctioned standard or an industry consensus in this area.
The only problem is that “broadly adopted standards” already exist for HTML email. They are called web standards, and almost every email client on the market meets these standards.