Ok I understand what you mean.

I think the only conditions for test failure are (A) $BROWSER is set and (B) $BROWSER is set to an actual executable that should be caught by ``register_standard_browsers`` prior to execution reaching the last if clause of ``register_standard_browsers`` (``if 'BROWSER' in os.environ:``) - those two conditions should be sufficient for execution to reach the offending line of code.

I'll see if I can cook up a test for that.  I think the right way to do this is to monkeypatch ``os.environ`` and ``shutil.which``, am I right?