fieldset element at XHTML Basic 1.1 and XHTML Print 1.0

I have tried to validate my web pages against XHTML Basic 1.1 or in
XHTML Print 1.0, and I **ONLY** found the following problems.

In XHTML Basic 1.1, fieldset is not a valid child of div. Why?

In XHTML Print 1.0, elements like fieldset, legend and optgroup are
not allowed. Steven kindly told me that XHTML Print 1.0 uses the Basic
Forms module from XHTML Modularization (which does not recognize such
elements). Can someone explain why these elements were rejected from
the Basic Forms module from XHTML Modularization?

Thanks.

Vicente Luque Centeno
Dep. Ingeniería Telemática
Universidad Carlos III de Madrid
http://www.it.uc3m.es/vlc