Not that I know of, and in fact real-live HTML emails often contain complete gibberish (made-up tags, etc.) that doesn't even validate; we have a bunch of fix-u

Not that I know of, and in fact real-live HTML emails often contain complete gibberish (made-up tags, etc.) that doesn't even validate; we have a bunch of fix-up rules for this stuff. CSS is a complicating factor as well: some CSS is unsafe and must be stripped, which requires a real (error-tolerant) CSS parser to go along with your real HTML parser. And identifying remote images: you'd think that was easy right? But do you know how many ways there are to reference a .png image in HTML and CSS? At one point Facebook emails were even hiding web bugs in BGSOUND tags -- sneaky!