We're slowly moving from a historical model based on layered “levels” of specs, to the living standard model actually implemented by browsers. As such, the code is kind of a mess.
Summary: new features go in lib/jsdom/living and follow the code style there; modifications to existing features will require some spelunking to find out what to modify in-place.
A lot of the main implementation is in lib/jsdom/level1 and lib/jsdom/level2. (That includes things that didn‘t appear in the original DOM Level 1 and Level 2 specs, just because the code was located there and we had to patch it.) We’re trying to avoid adding new code there, but patching old code is often still required.
New features generally go in the lib/jsdom/living folder, in nice small files, with a clear coding style enforced by JSHint.
We're planning to fix this whole situation with a multi-stage process:
lib/jsdom/browser and lib/jsdom/level3, as well as the more substantial body of code in lib/jsdom/level2, into lib/jsdom/level1. This will contain the “historical” portion of the jsdom codebase.lib/jsdom/level1 and into lib/jsdom/living, cleaning up the code style and spec compliance as we go.