[tacostakohashi]

As well as poking around in the source code, do not discount non-source based approaches.

For example:

* Run chromium using strace, ltrace, gdb to see what's going on at runtime.

* Do some experiments / reverse engineering, treating the application / source code as a black box. Try different HTML input, inspect the DOM in chrome, possibly automate this process via selenium or something, and discover the runtime behavior of the algorithm that way.

The thing to keep in mind is that, for all you know, the DOM building algorithm is split across thousands of source files, or is in fact in some dependency and not in chromium itself, or is split across both. Presumably there is some particular aspect of the DOM building that you are interested in, so experiment with how that works, instead of trying to find / understand the entirety of chromium DOM building.