Xhtml 1.1 doesn't seem to have the iframe module in its standard dtd. It should be possible to add the iframe module yourself.
However, I'm having a hard time finding easy-to-read documentation or a simple dummy tutorial on how-to expand the xhtml1.1 dtd with the iframe module. I'm not a newbie on html, but I don't quite grasp xhtml modularisation.
Therefore, 2 questions:
1) Do I need to expand the xhtml1.1 dtd with the iframe module to use an iframe in xhtml 1.1 or not?
2) If so, how do I do that? If not, is there another way?