Comment by runjake

Here's a rough break down, formatted best I can for HN:

  Safari (closed source)
   ├─ UI / tabs / preferences
   ├─ macOS / iOS integration
   └─ WebKit framework (open source) ~60%
        ├─ WebCore (HTML/CSS/DOM)
        ├─ JavaScriptCore (JS engine)
        └─ Web Inspector