I'm not seeing pytest complain in strict mode when filtering by markers that I haven't specified.
e.g. pytest --strict -m marker_that_i_tyoped_wrong mytests/
This will complete quite happily, deselecting all tests.

It would be nice if the behaviour were able to be changed, perhaps by another flag (to avoid causing sadness to anyone relying on the current behaviour), such that if the mark expression contained invalid marks it would cause a failure in the same way that a test containing invalid marks would.