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

Transparency is a presentational property and belongs in CSS,  etc., not
HTML.

> And here is my problem: on a website i have small iframes with dynamic

Note that one of the purposes of the split between transitional and
strict is to encourage the move away from the use of all sorts of frames,
as frames are incompatible with the concept that all resources can be
identified by URLs.

To some extent, I think I would have to ask why there ever was an
XHTML 1.0 transitional, as people wanting to stick to legacy features
should be using HTML, not XHTML.  There is no benefit in XHTML until
one starts serving as XML, and browsers that allow this usually have
good CSS support.