Indicates if an input element's value is valid.
The valid property indicates if an element's value satisfies
all conditions to pass a validation check. This includes basic validity
conditions, such as whether the element has a value if it is required,
and those specified in its onCustomValidation()
event handler, if you defined one.
Note that validations other than required, including custom validations, are not run on input elements when they don't have a value.