API for Creating Builds that Only Include Features Being Used

[jfftck]

Describe the new feature:
It would be nice to have an API that could include only the HTML and CSS features being used to create a custom build, this could replace the large Electron builds for desktop apps. Since this project is smaller, it would make a better base to create a slimmer runtime. Along with this, there should be an option for the build to disable the large memory cache, since most apps shouldn't need to grow much larger than when they start up.

This would fit nicely with the decoupled JavaScript runtime plans.

[mrobinson]

This could make sense, but I think it should be guided by measurement. Layout, is one of the more lightweight parts of Servo. You can observe this by compiling Servo yourself. The script crate takes much more time to compile than the two layout systems we have. Turning off CSS features is likely not going to gain you very much.

[sagudev]

If we can somehow make perfs to take effect in compile time as opt-in (so they would still work in runtime, but if you disable them in compile time you do not get it compiled so runtime pref would not work).