When linking to a file or website in the resources options, you can specify the parameter "Keep page navigation visible on the same page". If you check it, it will embed the file in an iframe. But iframes are not supported in XHTML 1.0 Strict, thus they need to be replaced.

The new form of specifying something similiar to the <iframe> tag is to use the <object> tag. Here is an example:

<iframe id="embeddedhtml" src ="http://www.ashcombe.surrey.sch.uk/MM_french/1a_self_family/self_family_fset.htm" width="100%" height="600"></iframe>

should be

<object id="embeddedhtml" type="text/html" data="http://www.ashcombe.surrey.sch.uk/MM_french/1a_self_family/self_family_fset.htm" width="100%" height="600"></object>

I have tested it in Firefox 3.0.4, Safari 3.2.1 and Internet Explorer 7 (IE7 asked me to accept the blocked content) and it works perfectly. If you want to tested in other browsers, it will be helpful.