I just tried validating the page I'm working on as

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

and it fails on the PayPal form:

<form target="paypal" ... /> there is no attribute "target"

<input type="hidden" name="cmd" value="_cart"...