According to 15 U.S.C. 7704 §5(a)(5):

INCLUSION OF IDENTIFIER, OPT-OUT, AND PHYSICAL ADDRESS IN COMMERCIAL ELECTRONIC MAIL.—

(A) It is unlawful for any person to initiate the transmission of any commercial electronic mail message to a protected computer unless the message provides—

(i) clear and conspicuous identification that the message is an advertisement or solicitation;
(ii) clear and conspicuous notice of the opportunity under paragraph (3) to decline to receive further commercial electronic mail messages from the sender; and
(iii) a valid physical postal address of the sender.

When my text-based mail client receives an HTML-only email message, it tries to render the HTML as text. It’s sometimes a jumbled up unreadable heap of garbage because the HTML is malformed and relies on a forgiving/tolerant rendering engine. Even when the HTML is well formed, hyperlinks are not exposed in the text rendered. E.g. a msg will say “to unsubscribe and stop receiving emails, update preferences here.”

Where is “here”? That is just raw text to me. Sure, an advanced user can do a number of things to dig up that link. But I doubt that would pass the legal standard of “clear and conspicuous”.

Anyone have confidence either way whether HTML-only spam is legally actionable on this basis?

(update) I should mention the most annoying offenders-- corporate senders (e.g. banks) that attach a plaintext MIME part, but then the motherfuckers use it to just say (in so many words) “You need to update your software”. This makes it extra difficult to see the content of the message because the text mail client of course shows the text MIME part by default.