1     <form> 2         <input type="text" name="username"/> 3         <input type="submit" /> 4     </form> 5     <script type="text/javascript" src=http://www.mamicode.com/"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>