How can I be notified when an element is added to the page?

Question

I want a function of my choosing to run when a DOM element is added to the page. This is in the context of a browser extension, so the webpage runs independently of me and I

Answer 1

The actual answer is "use mutation observers" (as outlined in this question: Determining if a HTML element has been added to the DOM dynamically), however support (specifically on IE) is limited (http://caniuse.com/mutationobserver).

So the actual ACTUAL answer is "Use mutation observers.... eventually. But go with Jose Faeti's answer for now" :)