How to make loaded html document use only it's own css stylesheet?

How to make loaded html document use only it's own css stylesheet?

Hi, I have a navigation bar on the left and when I click one of its buttons it loads another html doc in a div on the right. The problem is that the loaded document uses the main css file from index.html and I only want it to use another sub.css file for example.

Is there anyway to make it use only the linked sub.css file or do I just have to add lots of additional classes on the main.css file?
Here's the function:
  2. $("a").click(function () {
  3.     event.stopPropagation();
  4.     event.preventDefault();
  5.     $("html").velocity("scroll", {
  6.         duration: 600,
  7.         easing: "easeIn"
  8.     }).stop();
  9.     $('#side-txt').load(this.href);
  10. });