MathJax uses <script type="math/tex"> elements to store uncompiled math sections. When I was fiddling around to see if I could get MathJax working with PrinceXML in its current form, I found that the MathJax library would throw an error when it tried to access element.src, where element was one of the above script elements (which, indeed, has no src attribute). According to a discussion on stack overflow at http://stackoverflow.com/questions/12617028/should-a-script-tags-src-attribute-be-null-or-the-empty-string-if-missing/12633628#12633628, the correct behaviour is to return an empty string if the src attribute is missing from the tag, as it is defined as non-nullable in the standard (and this is the behaviour which occurs in most popular browsers)

As it happens, we have just fixed this issue and it will be released shortly as Prince 8.1 rev 4.

So does that mean that MathJax will work in 8.1r4?

No, I think there are more JavaScript DOM features that it requires.

@t.vogels, you may look here for a workaround.