Add support for detection indeterminate checkboxes
Add support for detection indeterminate checkboxes

NielsLeenheer/html5test,mpopp75/html5test,ya7lelkom/html5test,chiefcll/html5test,ya7lelkom/html5test,NielsLeenheer/html5test,mpopp75/html5test,chiefcll/html5test,ya7lelkom/html5test,mpopp75/html5test,NielsLeenheer/html5test,chiefcll/html5test