I use Gnus in Emacs as my email client. I've got it set up to render plain text if available, but if the mail is only HTML, it will fallback to use Emacs' shr (Simple HTML Render) library. It's basic support, but works well enough most of the time to make the mail readable. Worse comes to the worse, I just open the HTML in a web browser. If I get to that stage, it's almost always spam or something else not worth reading.