<iframe> content model

To the attention of validator.w3.org/nu staff.
Recently checked an XHTML version of a polyglot document with an <iframe>.
The document itself had previously been checked as HTML with no errors
neither with markup content, nor with plain text content.
In the XHTML document, when the content of the iframe was markup, it was
flagged as error and the suggestion was to use "Text that conforms to the
requirements given in the prose". Actually my markup was <b>source</b>, so
as far as I can understand from the prose it had to be correct. Then, when
I removed the <b> tag, the document itself was marked as error-free. But
the specification states, as content model for <iframe> in XML (and
therefore in XHTML) that this element must be empty. So it had to flag both
cases as the same error, while the requirements given in prose should be
valid only in HTML (no test of this case anyway).
Regards.
*Andrea Rendine*