Many html mail senders just use plaintext like this:

"Please view the HTML version of the e-mail"

Which is annoying, because I have a preference set for plain text if it is included. If a sender doesn't have a plaintext alternative, they should not include any of this garbage plaintext, that just confuses the preference logic in MUA.

Anyway, it shows that we may start seeing:

<p>You need AMP supporting MUA.</p>

as a html "alternative".