Servo is MPL not GPL licensed.

And even even that's a bit of a problem for the Servo project, as it makes it hard to move code between Servo crates and other Rust ecosystem crates (which are typically MIT/Apache2.0), which means that Servo is it's own "island" somewhat disconnected from the rest of the Rust ecosystem.

(there's nothing inherently wrong with the MPL, but it being a different and incompatible license to the rest of the ecosystem does cause problems)