为什么提交用户名和密码后 点击退回按钮提交的用户名和密码还在文本框？

退回后就是图片里面的样子 怎样让他自动清空呢

<code>


    <meta charset="utf-8">
    <title>
        
    </title>
    <style type="text/css">
    div input{display: inline-block;width: 200px;height: 20px;border:1px solid #ccc;}
    div .span1{display: inline-block;width: 90px;height: 20px;text-align: left;margin-top: 20px;}
    div #sp1,#sp2,#sp3,#sp4{display: inline-block;width: 160px;height: 20px;}
    </style>
    <script src="ajax.js"></script>
    <script type="text/javascript">
        window.onload=function(){
            var oInput=document.getElementsByTagName('input');
            var oIpt1=document.getElementById('ipt1');
            var oIpt2=document.getElementById('ipt2');
            var oIpt3=document.getElementById('ipt3');
            var oIpt4=document.getElementById('ipt4');
            var oSp1=document.getElementById('sp1');
            var oSp2=document.getElementById('sp2');
            var oSp3=document.getElementById('sp3');
            var oSp4=document.getElementById('sp4');
            var re=/^[\w]{6,12}$/;
            oIpt1.onblur=function(){
                if(oIpt1.value==''){
                    this.style.border='1px solid red';
                }else{
                    ajax("nameajax.php?id="+oIpt1.value,function(str){
                    oSp1.innerHTML=str;
                    })
                }
                
            }; 
            oIpt2.onblur=function(){
               if(oIpt2.value==''){
                   this.style.border='1px solid red';
               }else{
                     if(oIpt2.value.match(re)){
                         oSp2.innerHTML='密码合法';
                     }else{
                         oSp2.innerHTML='密码不合法';
                     }
                  
               }
            }
            oIpt3.onblur=function(){
                if(oIpt3.value==''){
                   this.style.border='1px solid red';
               }else{
                   if(oIpt2.value==oIpt3.value){
                       oSp3.innerHTML='密码正确';
                   }else{
                       oSp3.innerHTML='密码不一致';
                   }
               }
            };
            oIpt4.onblur=function(){
                if(oIpt4.value==''){
                    this.style.border='1px solid red';
                }else{
                    ajax("emailajax.php?id="+oIpt4.value,function(str){
                    oSp4.innerHTML=str;
                    })
                }
                
            }; 
       };
    </script>


<div>
    <form action="reg.php" method="post">
        <span class="span1">用 户 名:</span><input type="text" id="ipt1" name="username" autocomplete="off"><span id="sp1"></span><br>
        <span class="span1">密　　码:</span><input type="password" id="ipt2" autocomplete="off"><span id="sp2"></span><br>
        <span class="span1">确认密码:</span><input type="password" id="ipt3" name="password" autocomplete="off"><span id="sp3"></span><br>
        <span class="span1">邮　　箱:</span><input type="text" id="ipt4" name="email" autocomplete="off"><span id="sp4"></span><br>
        <button type="submit">提交</button>
    </form>
</div>


</code>

<code><?php $pdo=new PDO("mysql:host=localhost;dbname=t1","root","");
$stmt=$pdo->prepare("insert into user(username,password,email) values (:username,:password,:email)");
$stmt->execute($_POST);
?></code>

回复内容：

退回后就是图片里面的样子 怎样让他自动清空呢

<code>


    <meta charset="utf-8">
    <title>
        
    </title>
    <style type="text/css">
    div input{display: inline-block;width: 200px;height: 20px;border:1px solid #ccc;}
    div .span1{display: inline-block;width: 90px;height: 20px;text-align: left;margin-top: 20px;}
    div #sp1,#sp2,#sp3,#sp4{display: inline-block;width: 160px;height: 20px;}
    </style>
    <script src="ajax.js"></script>
    <script type="text/javascript">
        window.onload=function(){
            var oInput=document.getElementsByTagName('input');
            var oIpt1=document.getElementById('ipt1');
            var oIpt2=document.getElementById('ipt2');
            var oIpt3=document.getElementById('ipt3');
            var oIpt4=document.getElementById('ipt4');
            var oSp1=document.getElementById('sp1');
            var oSp2=document.getElementById('sp2');
            var oSp3=document.getElementById('sp3');
            var oSp4=document.getElementById('sp4');
            var re=/^[\w]{6,12}$/;
            oIpt1.onblur=function(){
                if(oIpt1.value==''){
                    this.style.border='1px solid red';
                }else{
                    ajax("nameajax.php?id="+oIpt1.value,function(str){
                    oSp1.innerHTML=str;
                    })
                }
                
            }; 
            oIpt2.onblur=function(){
               if(oIpt2.value==''){
                   this.style.border='1px solid red';
               }else{
                     if(oIpt2.value.match(re)){
                         oSp2.innerHTML='密码合法';
                     }else{
                         oSp2.innerHTML='密码不合法';
                     }
                  
               }
            }
            oIpt3.onblur=function(){
                if(oIpt3.value==''){
                   this.style.border='1px solid red';
               }else{
                   if(oIpt2.value==oIpt3.value){
                       oSp3.innerHTML='密码正确';
                   }else{
                       oSp3.innerHTML='密码不一致';
                   }
               }
            };
            oIpt4.onblur=function(){
                if(oIpt4.value==''){
                    this.style.border='1px solid red';
                }else{
                    ajax("emailajax.php?id="+oIpt4.value,function(str){
                    oSp4.innerHTML=str;
                    })
                }
                
            }; 
       };
    </script>


<div>
    <form action="reg.php" method="post">
        <span class="span1">用 户 名:</span><input type="text" id="ipt1" name="username" autocomplete="off"><span id="sp1"></span><br>
        <span class="span1">密　　码:</span><input type="password" id="ipt2" autocomplete="off"><span id="sp2"></span><br>
        <span class="span1">确认密码:</span><input type="password" id="ipt3" name="password" autocomplete="off"><span id="sp3"></span><br>
        <span class="span1">邮　　箱:</span><input type="text" id="ipt4" name="email" autocomplete="off"><span id="sp4"></span><br>
        <button type="submit">提交</button>
    </form>
</div>


</code>

<code><?php $pdo=new PDO("mysql:host=localhost;dbname=t1","root","");
$stmt=$pdo->prepare("insert into user(username,password,email) values (:username,:password,:email)");
$stmt->execute($_POST);
?></code>

浏览器问题吧，我之前用火狐好像也会出现这个问题。
在form里面添加
js代码添加
$("input[type=reset]").trigger("click"); //jquery提供的方法
或者遍历表单内的元素清空内容。
我这里有个链接你可以参考下http://www.cnblogs.com/shanlin/archive/2014/07/17/3850417.html

现在浏览器一般有自动保存用户名和密码的行为，可以使用js将用户名和密码设置成空：
var oIpt1=document.getElementById('ipt1');
var oIpt2=document.getElementById('ipt2');
oIpt1.value = '';
oIpt2.value = '';

使用ajax 提交登陆，只要就不会存在后退了