JavaScript standard base

[littledan]

TC39 is working on infrastructure for built-in modules, which could be used to expand the standard library. Complementary to that, there are a number of things which are shared between JavaScript embedders, which are outside of the scope of TC39, and work well that way.

Something we don't have is a listing of what these libraries are, what's expected from an environment that conforms to the listing, and what sorts of deviations are permitted (e.g., omitting the origin model when outside of a browser).

Should we try to define such a "JavaScript standard base"? Perhaps as part of the project of this repository?