> I'm aware of that but my point is that most employers would not allow that since the employer is also liable if it under-withheld for a W2 employee.

AFAIK, they aren't liable for under withholding if they withhold correctly according to the W-4 signed by the employee, even if the W-4 claims things (e.g., "exempt" when the employee does not meet the qualifications) that are not accurate.