Do you have more specifics, and/or have you advised them of the difficulties? It seems to use the same "mach" build process as does firefox, and I build FF developer edition regularly (not every day, but damn near): https://github.com/servo/servo#normal-build

While investigating this, I have deep sympathies for whoever has to work with that taskcluster silliness because yikes that is some A++ grade obfuscation as compared to a .gitlab-ci.yml or .circleci or even the .travis.yml they migrated off of

This makes me feel better about abandoning my own plans to do the same.

Servo uses SpiderMonkey as its JS engine, and supports some DOM apis.

you probably wouldn't want it anyway. Using Servo only for rendering and using Rust for everything else would be preferable.