I would advise the client NOT to do HTML e-mails for the simple reason many servers (like mine) are set up to auto-block them as spam; since no legitimate e-mail I would ever expect from someone has the need to have HTML in it. It's a great way to filter out the SPAM.
As to doctype? NONE. Why NONE? Because as mentioned external stylesheets don't work with them and inlined CSS generally doesn't work right either; The most recent HTML specification that e-mail clients even support is HTML 3.2 -- at which point you don't need the doctype.
Which IMHO is just another reason NOT to do HTML e-mails in the first place.
Which again is so meaninglessly vague after that recent 'change' that it's probably used less than it was before the change.
"Flag" -- what the devil does that even MEAN? JHVH you use wording like "report to moderator"... at LEAST as the title attribute. The nonsense about "report" or "warning" having a negative connotation is almost as silly as the high school I went to two decades ago; They handed out "E"'s because "F" had the negative connotation of Failure -- which is why the key on the report card read "E - Failure"...
Basically anyone who never saw the thread about the change is going to have no clue what that even IS! I bet johnuk was looking for the report button and couldn't even find it!