Life is good, using XHTML Strict with much success
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
...Then, bam! You toss a Vimeo video onto your page that breaks validation.
<iframe src="http://player.vimeo.com/video/XXXXX?byline=0&portrait=0" width="468" height="351" frameborder="0"></iframe>
No iframes allowed, sir!
So, what impact does it have on the way the page renders, or impact on the visitor? Why care about the validation of this iframe?
It doesn't have an impact on the page itself. Unless you are worried about validation, I would stay with the transitional doctype:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The iframe validation is solely up to you. Here is a list of elements that compare with each doctype.