tenfourfox/testing/web-platform/tests/editing

This suite 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.