为什么样式里面改不了高度

<div style="width:100%; height:30px; line-height:30px; text-align:left">    <div style="width:20px; height:30px; float:left; background-color:#ffffff; border-bottom:1px solid #999999; border-top:1px solid #ffffff;"></div>
    
    <div style=" width:120px; height:29px;float:left; position:relative; text-align:center;border:1px solid #999999; border-bottom:none">aaaaaa</div>
    
    <div style="width:20px; height:30px; float:left; background-color:#ffffff; border-bottom:1px solid #999999; border-top:1px solid #ffffff;"></div>
    
    <div style=" width:120px; height:29px;float:left; position:relative; text-align:center;border:1px solid #999999">bbbbbb</div>
    
    <div style="width:20px; height:30px; float:left; background-color:#ffffff; border-bottom:1px solid #999999; border-top:1px solid #ffffff;"></div>
    
    <div style=" border:1px solid #999999;width:120px; height:29px;float:left; position:relative; text-align:center">cccccc</div>
    
    <div style="width:100px; height:30px; float:left; background-color:#ffffff;border-bottom:1px solid #999999; border-top:1px solid #ffffff;"></div>
    
    </div>