While the demo might use Ajax requests jsFiddle will read them and send using the right MIME type.
It is important to have demo responses prepared for jsFiddle.
URL: /gh/get/response.{extension}/{github_tree}/ where:
Note
There is no JSONP functionality provided. One may achieve it using raw GitHub URL.
Content of the http://github.com/zalun/jsFiddleGithubDemo/raw/master/Demo/demo.response.html will be returned with text/html MIME type if this url will be loaded http://jsfiddle.net/gh/get/response.html/zalun/jsFiddleGithubDemo/tree/master/Demo/ using Ajax request.
Here it is working within the jsFiddle