[XHTML 2] 17 Clipping of embedded documents to viewport.

Dear HTML Working Group,

If the embedded resource is an XHTML 2 document that is styled with CSS, 
such that content is positioned at top:-100px;left:-100px, must this content 
be rendered by a CSS supporting user agent, or is it implementation 
dependant:

e.g.

<h1 style="position:absolute;top:0;left:0;">Donkeys</h1>
<p style="position:absolute;top:100px;left:0;" src="donkey.xhtml" />

and in donkey.xhtml
<p style="position:absolute;top:-100px;left:0">Chickens!</p>

Is the Chickens! rendered on top of the h1?  I realise this may be the 
domain of

(with appropriate other mark-up to make the documents valid)

Regards,

Jim Ley.