My coworker pointed out that when you're navigating through Youtube, you can go from page to page and the URL completely changes (i.e. it's not just changing stuff after the # anchor) but parts of the page, such as the side bar, remain constant (tampering with it with Inspect Element proves that it's the same DOM elements, not new copies each time)
I guess that's a thing, now! With appropriate js shims for older browsers. I don't have a reason to implement a proof of concept for myself right now, but it's a good thing to know about.