gecko-dev/testing/web-platform/tests/editing

Most of this directory tests conformance to the editing spec written long ago
by Aryeh Gregor.  Nobody actually implements the spec, but the tests are still
useful for regression testing.  The files in data/ were generated from a
JavaScript implementation of the specification using a complex procedure that
can't actually be replicated right now as-is.  Editing them manually is
possible, but they're not really meant to be human-readable.  If anyone is
interested, it would be possible for Aryeh to get the test generation procedure
working again.  Or you could look into the repository history and figure out
how to do it yourself, if you're brave.

There is also a directory other/ that contains additional editor-related tests.
They aren't necessarily based on any specification, but try to specify sensible
behavior, and are meant to be helpful with regression testing for existing
implementations and finding bugs in new implementations.