Keep running into issues where browser engineers write spec IDL that violates the IDL spec in whatever ways their binding implementation does not enforce. If we're lucky a second implementor, with a binding implementation with different holes, tries to implement before it ships
The prose issues, by the way, tend to be caused by a mismatch between what the IDL spec says the IDL values produced should look like and what bindings in some browser do. The spec prose gets written based on the latter, not the former. This is hard to catch via a verifier.