I think I got it. Let's say that I have something like the following:
The following has the right idea:
myObject.body.style.border = 'none';
myObject.body.style.margin = '0';
myObject.body.style.padding = '0';
myObject.body.style.overflow = 'hidden';
in http://www.mySite.com/someURL (in retrospect, this seems rather obvious given the information). This gets rid of border and scrollbar AND enables to embed an external html while the parent html stays in XHTML Strict compliant.
(I still need to set target to "_top", but I'm almost there...)