Maybe WebKitGTK. They provide the only library that you can use in a reasonable way to build your own browser. But it has so many bugs and performance issues, it’s sad. As soon as they catch up, some new fancy web standard emerges and they quickly fall behind again.

(But to be honest, I’d rather send the money to some hypothetical hardware manufacturer and tell them to make us some sane CPUs and mainboards. :-) Well documented, free software, no out-of-order, no hidden software, …)