nav {
		  position: absolute;
		  top: 0;
		  left: 0;
		}
		
		.nav_links {
		  display: none;
		  position: fixed;
		  top: 0;
		  left: 0;
		  background-color: lightblue;
		  overflow: auto;
		  border-radius: 10px;
		  padding-top: 10px;
		  padding-left: 20px;
		  padding-right: 10px;
		  border: 2px solid blue;
		}
		
		nav a {
		  display: block;
		}
		
		nav a:hover {
		  background-color: lightyellow;
		}
		
		body {
		  padding-top: 20px;
		  background-color: lightgrey;
		}
		
		#menu_button { 
		  position: fixed;
		  top: 0;
		  left: 0;
		}
		
		nav:hover .nav_links { 
		  display:block;
		}
		
		td {
		  border: 1px solid gray;
		  vertical-align: middle;
		}
		
		.tab { 
       display:inline-block; 
       margin-left: 40px; 
}