Java jsp单独运行left里面的有四个按钮，运行index只有两个按钮这是为什么啊

运行indext.html时左边只有两个按钮

img


单独运行left.html时是四个按钮

img


index.html里面的代码


<!DOCTYPE html>
<!--
To change this license header, choose License Headers in Project Properties.
To change this template file, choose Tools | Templates
and open the template in the editor.
-->
<html>
    <head>
          
        <title>Book information system</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        
                <style>
            #top{
                width: 100%;
                height: 100px;
               
            }
            #left{
                width: 20%;
                height: 400px;
            }   
            #content{
                width: 78%; 
                height: 400px;
            }
        </style>
        
    </head>
    <body>
    
        <div>
            <iframe id="top" name="top" src="top.html"></iframe>
            <iframe id="left" name="left" src="left.html"></iframe>
            <iframe id="content" name="content"></iframe>
        </div>
    </body>
</html>

lef.html里面的代码


<!DOCTYPE html>
<!--
To change this license header, choose License Headers in Project Properties.
To change this template file, choose Tools | Templates
and open the template in the editor.
-->
<html>
    <head>
        <title>TODO supply a title</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body style="background: seashell;">
       <div>图书出版管理功能：</div>
       <p><a href="book/bookinfo.jsp" target="content"><button type="button">查询出版图书</button></a></p>
       <p><a href="book/bookAddInfo.jsp" target="content"><button type="button">添加出版图书</button></a></p>
       <p><a href="bookSale/bookSaleinfo.jsp" target="content"><button type="button">查询出版图书的销售信息</button></a></p>
       <p><a href="bookSale/TJZXL.jsp" target="content"><button type="button">统计某图书的总销量</button></a></p>
    </body>
</html>

left的高度导致下面的没显示出来，可以试着加大高度。

img