WHY not allowtransparency attribute?? please explain

I really dont undersand why you guys do not allow the allowtransparency
attribute! (in XHTML transitional)

Can somebody explain why it isnt allowed in XHTML? The browsers dont seem to
have any problem with it! FireFox, IE, Safari... so why not? Wich browsers
dont support this??

And here is my problem: on a website i have small iframes with dynamic
textual content,  absolutly positioned upon and on top of a changing/dynamic
graphic background layer, wich extends all through the browsers viewport.

So one can scroll the content-text of the small iframe, while the background
layer with all the graphic stuff is visible, even through the iframe with
its text content. Thats is exactly what the "allowtransparency attribute"
does perfecty! And it works, crossbrowser. Untill i see i cant validate
because you people dont allow the atribute.

And NO, other options are no good:
a fixed background on the iframe content wont work, because it would not
change along with the dynamic graphic background on the layer beneath the
iframe. So thats NO option.
And NO, opacity upon the iframe or surrounding div is not what i want,
because the TEXT will be also getting transparent, so thats no option.
And YES, the iframe is needed, and is not replacible with a div etc, because
i have .PHP pages wich are dynamically loaded into the iframe, and there is
no other way than using iframes to accomplish that (in order to not having
to load the entire website all the time).

And YES, its all valid XHTML, all valid CSS etc, no other errors etc, so no
need to look into the rest.... the only thing is you people not allowing the
allowtransparency attribute!

So please explain WHY not??

Clif Grep
Netherlands