Low level browser functions

I've been poking around the TinyMCE code and I notice that when you click a toolbar button, like the "B" for bold,  the first TinyMCE code entered is the defaultNativeHandler(evt), where the evt object includes the x and y of where you clicked and also, under "target" the outerHTML for the element that was at that location:

This tells me that the browser itself handled the mouse click event and figured out itself what HTML was at that location, and then passed a nice evt object with all that information filled in to the TinyMCE code.  Could someone point me to some documentation on what services browsers provide at this low level or maybe provide an explanation?  I think I've heard that browsers will also perform bold and italic operations for upper code.

Thanks for any information.

Steve