Comment by polyglotfacto

I don't think it's all copy/pasted; it is quite an original byzantine architecture.

You're right that lots of code appears only used in unit-tests, of which there is an enormous amount(making it hard to tell whether what is being tested makes sense). In Servo we don't have a single line of unit-tests in the script component, because all of it is covered by the WPT integration test suite shared with all other engines...