input元素required属性无效

$(document).ready(function(){
 $("form").submit(function(e) {
 var ref = $(this).find("[required=required]");
 alert('ref');
 $(ref).each(function(){
 if ( $(this).val() == '' )
 {
 alert("Required field should not be blank.");
 $(this).focus();
 e.preventDefault();
 return false;
 }
 }); return true;
 });
});