DHTMLXgrid Doctype problem in IE9

I’m facing the problem that my DHTMLXgrid does not load when the doctype is:

My code has the right doctype (so the DHTMLXgrid works great in my testing environment) but it needs to run in an iframe. Now IE9 has the bad habit to force the doctype from the parent document to its child, even though the child document has its own doctype.

Is there a solution to have a working DHTMLXgrid with the doctype as stated above? Do we need a newer version of DHTMLXgrid?

Anyone? It would be great if there was a solution to the DOCTYPE problem.