Validation notes

• invalid fields prevent submission
• hidden fields are validated
• disabled fields are NOT validated
• an invalid field can be disabled, and will not then prevent submission
• invalid events are thrown when form submission is attempted
• submit event never occurs when invalid fields are present
• invalid events are thrown when .checkValidity() is called
• there is no event when submit is cancelled due to invalid fields (unlike xforms submit-error event)
• there is no event when a field becomes valid (unlike xforms valid event)
• :valid and :invalid psuedo-classes are updated on input