The KHTML developers have complained that it's hard to integrate work done by Apple's Safari team, since all they get are periodic code dumps with no history. I agree that the Safari/KHTML relationship doesn't seem to be one of full development peers, but the lack of history claim is not true. WebCore 315 and WebCore 413 both include very detailed (checkin by checkin) changelogs of what went into them.
In fact, these ChangeLogs make interesting reading even for non-browser developers. For example, there are lines such as "crash in ApplyStyleCommand::applyBlockStyle pasting contents of webpage into Mail or Blot". It's well known that Mail.app uses WebCore in
contentEditable mode for composing, but the "Blot" application is new. Based on its name, one might be inclined to suspect that it's a blog authoring tool. There's also a lot of self-reviewing (search for "reviewed by me") which wouldn't fly at other development shops (*cough*).