1     <form>
 2         <input type="text" name="username"/>
 3         <input type="submit" />
 4     </form>
 5     <script type="text/javascript" src="jquery.js"></script>
 6     <script type="text/javascript">
 7         var isUsernameRequired = document.forms[0].elements[‘username‘].required;
 8         alert(isUsernameRequired); //字段是否有required检验
 9 
10         var isRequiredSupport = ‘required‘ in document.createElement(‘input‘);
11         alert(isRequiredSupport);   //浏览器是否支持验证
12     </script>