Initial content for an iframe

Hi everybody!
I recently had a huge maintenance session on a website project which makes
use of <iframe>s and made use of the srcdoc attribute.
The maintenance session was due to a change in the markup, but it was also
meant to insert semantic markup throughout the site pages.
I initially studied the implementation of iframe@srcdoc in a way similar to
the one in the spec, i.e. as a protected context for user-added content.
But during the initial elaboration I encountered several problems, which
doubled during maintenance. This is made worse by the fact that I strongly
want to integrate semantic markup in the comments, so that they become part
of the context's owner document. In addition to this, I had some trouble
because of the fact that my pages are meant to be XHTML. I was forced to
abort the entire project.
So I asked myself: why has it been decided that "source markup" for an
iframe had to be so difficult in syntax and poor as integration? I'd like
to hear someone else's thoughts, and I'd also like to ask if anybody has
thought to replace @srcdoc with the actual content for <iframe>.
Thank you.
A.R.