[Bob Williams]

Actually, if you're talking about proper Mac behavior, the correct method when dealing with document windows, as laid out in Apple's well-thought-out Human Interface Guidelines many years ago, is to open each subsequent window offset horizontally and vertically from the previous enough that you can read and click on the preceding windows' title bars.

You can see this behavior in such well-designed third-party apps as BBEdit, Interarchy, and, yes, OmniWeb. With Apple's apps, TextEdit, Mail, Address Book, and even the often-obnoxious Finder work this way. Apple's not completely with the game, though - Terminal does it right sometimes, but other times, it picks seemingly random positions. Note, though, that to see this behavior in its full glory in some apps (like OW), your window must be small enough to shift in both directions; if it's not, then the shift will only occur in one or neither direction. (In other apps, subsequent windows are made a bit smaller to allow the shift. Which is better is very debatable and probably best chosen on an app-by-app basis.) Of course, other factors are considered, as well, such as whether the title bar can be reached in the proposed position.

I think my ideal for a web browser is to save a user-defined size, then offset new windows appropriately but without changing the size (which means at the upper limit, which is full-screen, you simply end up with another full-screen window). And, this is exactly what OW does. I don't see how it's unpredictable or peculiar - it's in accordance with recommendations designed through research to maximize usability.