导航栏不对字体显示不出

代码中的字体颜色设置成白色，为什么不会再导航栏显示，另外请老师帮忙看一下代码有哪些改进之处，O(∩_∩)O谢谢

<!DOCTYPE html>
<html>
<head>
 <meta charset="UTF-8">
 <title>3-3编程题</title>
 <style type="text/css">
  *{
   margin:0;
   padding:0;
  }
  body{
   min-width: 1000px;
  }
  .header{
   float:left;
   width: 100%;
   background-color: black;
   height:100px;
   line-height: 100px;
   /*为什么我这里不设置字体颜色会显示不出字体？*/
   color:white;   
   position: fixed;
   z-index: 99999;/*使导航栏显示在内容上方*/
   top:0px;
  }
   div.header img{float: left;}
   div.header p{
    float: right;
   }
     div.header p span{
      cursor:pointer;
     }
     div.header p span:hover{
      color: orange;
     }
  .footer{
   float:left;
   width: 100%;
   background-color: black;
   height:50px;
   text-align: center;
   line-height:50px;
   color:white;
   position: fixed;
   bottom: 0;
  }
  div.footer p span{
   cursor: pointer;
  }
  /*圣杯布局*/
  .container{
   padding: 100px 250px 0 350px;   /*加入100px上内边距，使文字不被遮盖*/
  }
  /*圣杯布局核心*/
  .left,.right,.middle{
   position: relative;
   float: left;
   min-height:1000px;
   padding-top: 100px;
  }
  .middle{
   width: 100%;
   background-color: #ffc0cb;
   text-align: center;
  }
  .left{
   width: 350px;
   background-color:#ffefdb; 
   margin-left: -100%;
   left:-350px;
  }
  .right{
   width: 250px;
   background-color: #add8e6;
   margin-left: -250px;
   right:-250px;
  }
  /*内容样式处理*/
  div.left table{
    width: 100%;
    float: right;
    position:absolute;
    top:100px;
  }
  div.left table td{
   margin: 10px;
   padding: 10px;
     }
  div.left table tr td span{
    background-color:#f06; 
   }
  .denglu{
   width: 100%;
   float: left;
   padding-left:10px;
   position:absolute;
   top: 100px;
  }
  h2{
   margin-bottom: 10px;
  }
  .form{
    width:200px;
               height:30px;
               margin-top:10px;
                margin-bottom:10px;
                font-size:10px; 
                border-style: none;
  }
  .submit{
   width:200px;
               height:35px;
               margin-top:10px;
                margin-bottom:10px;
               background-color:#ff0002;
               color:#fff;
               font-size: 14px;
               border-style: none;  }
 </style>
</head>
<body>
 <div class="header">
  <img src="logo.png">
  <p><span>课程</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>职业路径</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>实战</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>猿问</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>手记</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</p>
 </div>
 <!-- 这是页面的内容 -->
 <div class="container">
  <div class="middle">     <img src="6.jpg">
   
  </div>
  <div class="left">      <table>
         <tr><td><h2>课程推荐</h2></td></tr>
      <tr><td><span>职业路径</span>&nbsp;&nbsp;&nbsp;&nbsp;HTML5与CSS3实现动态网页</td></tr>
      <tr><td><span>职业路径</span>&nbsp;&nbsp;&nbsp;&nbsp;零基础入门Android语法与界面</td></tr>
      <tr><td><span>职业路径</span>&nbsp;&nbsp;&nbsp;&nbsp;iOS5基础语法与常用控件</td></tr>
      <tr><td><span>职业路径</span>&nbsp;&nbsp;&nbsp;&nbsp;PHP入门开发</td></tr>
      <tr><td><span>职业路径</span>&nbsp;&nbsp;&nbsp;&nbsp;Java入门开发</td></tr>
     </table>   </div>
  <div class="right">
     <div class="denglu">
       <h1>登录</h1>
            <form>
                 <input type="text" name="username" placeholder="请输入登录邮箱/手机号" class="form"><br>
                 <input type="password" name="password" placeholder="6-16位密码，区分大小写，不能用空格" minlength="6" maxlength="16" class="form"><br>
                 <input type="submit" name="submit" value="提交" class="submit">
            </form>
     </div>
  </div>
 </div>
 <div class="footer">
 <p><span>网站首页</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>企业合作</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>人才招聘</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>联系我们</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>常见问题</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span>友情链接</span></p>
 </div>
</body>
</html>

飒场就典兵 2017-09-05

源自：CSS网页布局基础 3-3 编程练习