1. html,body{height= 100%;}
2.  
3.  
4. body{font-family: sans-serif;}
5.  
6. nav{background: #000; height: 52px; width: 100%; position: fixed; top: 0; Left: 0,}
7.  
8. nav ul{list-style: none; margin: 0 auto; padding: 0; width: 100%; max-width: 1200px;}
9.  
10. nav ul ul{display: none; position: absolute; top: 100%; left: 0; width: 100%;}
11.  
12. nav ul li{position: relative; float: left; width: 20%; text-align: center;}
13.  
14. nav ul li:hover ul{display: block;}
15.  
16. nav ul li li{position: relative; float: none; width: 100%; border: none;}
17.  
18. nav ul li a{display: block; text-decoration: none; color: #FFF; padding: 15px 25px; font-size: 1.2em; box-sizing: border-box; border-right: solid 2px #000; border-left: solid 2px #000;}
19.  
20. nav ul ul li a{font-size: 1rem; border-bottom: solid 2px #000;}
21.  
22. nav ul ul li:first-child a{border-top: solid 2px #000;}
23.  
24. nav ul li:hover a{background: #FFF; color: #000; text-decoration: none; transition: 0.5s;}
25.  
26. nav ul li li:hover a{background: #000; color: #FFF;}
27.  
28. .cover{height:100%; border: 1px solid red;}