I create a simple harness.xhtml to load jQuery:
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
And then I create the file widget.html:
When I open harness.xhtml in Firefox 4 (either from a hosted location or using a file:// url), I get the following script error:
An invalid or illegal string was specified" code: "12
If I remove the <!DOCTYPE html> line from the widget.html, it loads as I would expect, there is no script error, and the browser displays "Got it!".
It appears that single <!DOCTYPE> declaration is sufficient to prevent firefox from loading the content dynamically.
Nothing in the docs suggests why this would be the case. In fact, the examples from the docs appear to have <!DOCTYPE html> declarations.