The Mozilla layout code uses frame objects to lay out the DOM on the screen. (These are entirely different from <:frame> nodes in the DOM). All Mozilla frames inherit from a single C++ abstract class nsIFrame. As part of a project I’m working on to separate the frame classes from XPCOM, I used dehydra to generate a graph of all the frame types in Mozilla.
What I really want for Christmas is a web-based interactive graph viewer for this type of content. I’ve seen a couple closed-source things in Java, but nothing really exciting or hackable.
Note: Viewing this graph in Safari won’t work, because the image is much larger than a single screen, and Safari doesn’t provide scrollbars for SVG that overflows… feel free to download it and view in inkscape, though, or get Firefox!.