I think you could argue that this is a bug. The trouble is that the browser returns an empty string to AngularJS if the contents is not a number:
In the numberInputType directive it doesn't consider empty strings to be invalid:
function numberInputType(scope, element, attr, ctrl, $sniffer, $browser) {
textInputType(scope, element, attr, ctrl, $sniffer, $browser);
ctrl.$parsers.push(function(value) {
var empty = isEmpty(value);
if (empty || NUMBER_REGEXP.test(value)) {
ctrl.$setValidity('number', true);
return value === '' ? null : (empty ? value : parseFloat(value));
} else {
ctrl.$setValidity('number', false);
return undefined;
}
});