make an drodown menu

1/*The css file*/
2nav {
3  background-color: black;
4  max-width: 300px;
5  margin: 10px;
6}
7nav li ul {
8  visibility: hidden;
9  height: 0;
10}
11nav > ul > li:hover ul {
12	visibility: visible;
13  	height: 100px;
14  	width: 60px;
15  	transition-delay: 0.5s;
16} 

1<!--on the html document-->
2<nav>
3	<ul>
4    	<li>
5        <a href="#">Hello</a>
6        	<ul>
7            	<li>World!</li>
8                <li>This is a dropdown</li>
9            </ul>
10        </li>
11  </ul>
12</nav>