To reproduce:

On the index demo, edit the markup to fill in the second field with a valid email address. Load that demo, type "abc" into the first field, tab to the next field, tab back, delete the value.

Expected: "This field is required" error.
Actual: No error

Originally reported as #244, but its really not specific to select elements. Reverted the fix from #396 in 5934f4d, as the caused a regression (#521).

This seems pretty tough to fix, as it scratches on the very basics of how the plugin figures out if a field should be validated or not. Haven't yet been able to come up with a better solution.