Since this is using Cocoa and one of the targets is MacOS, have you considered using the GNUstep framework {instead of|in addition to} gtk* for the Linux side of your browser project?
I'm targetting GTK because it's the most common toolkit and it is easy to interoperate from other languages. I will also attempt to do a Qt/KDE support at some point in the future, but C++ is more complicated with binary compatibility. GNUstep seems like something that not many users have installed.