[encoderer]

Which really should be trivial using most modern 3-Tier frameworks. They universally have a concept of "layout" vs "template" so in a pseudocode logic, all you need to do is tap into the render method and swap out an empty layout when responding to an XHR request.

[odiroot]

Exactly, however one can argue the number of your test cases (and cache entries) doubles. Still I think it's an elegant solution.