The things you want to omit have nothing to do with Servo is the point of my comment. Servo's modularity is irrelevant to removing webcam access. Which incidentally I believe you can already do in both Firefox and Chrome by disabling the MediaSource API altogether.
Servo isn't strictly just the HTML+CSS part, if you have a look at the project's sources.
Modularity is relevant because if stuff isn't modular and has code paths to deal with missing functionality, you'll have a much increased burden, aka a fork.
In Firefox (and maybe Chrome, don't use, so not sure) you can disable it in the configuration but that could easily be enabled again by some script or extension.
