CSS下拉菜单导航栏

.nav {
  list-style: none;
  margin: 0;
  padding: 0;
  position: relative;
  text-align: left; /* change to "center" or "right" to align differently */
  border-bottom: 10px solid green; /* bottom border */
  background: #ffffff;
  background: -moz-linear-gradient(top,  #ffffff 0%, #d8d8d8 100%); 
  background: -webkit-gradient(linear, left top, left bottom, color-     stop(0%,#ffffff), color-stop(100%,#d8d8d8));
  background: -webkit-linear-gradient(top,  #ffffff 0%,#d8d8d8 100%);
  background: -o-linear-gradient(top,  #ffffff 0%,#d8d8d8 100%);
  background: -ms-linear-gradient(top,  #ffffff 0%,#d8d8d8 100%);
  background: linear-gradient(to bottom,  #ffffff 0%,#d8d8d8 100%);
 }
.nav ul li{
  display: inline-block;vertical-align:top;
  }
.nav ul a{
  position: relative;
  display: inline-block;
  overflow: hidden;
  color: black; /* font color */
  text-decoration: none;
  padding: 8px 20px;
  font-size: 14px; /* font size */
  font-weight: bold;
  vertical-align: bottom;
 -webkit-transition: color 0.5s; /* transition property and duration */
 -moz-transition: color 0.5s;
 transition: color 0.5s;
 }

ul .nav a::before{
 content: '';
 color: white;
 display: block;
 position: absolute;
 width: 100%;
 height: 100%;
 background: green; /* tab background */
 left: 0;
 top: 110%; /* extra 10% is to account for shadow dimension */
 box-shadow: -2px 2px 10px rgba(255,255,255,.5) inset;
 border-radius: 15px 15px 0 0 / 12px 12px;
 -webkit-transition: top 0.5s; /* transition property and duration */
 -moz-transition: top 0.5s;
 transition: top 0.5s;
}

ul .nav a:hover{
 color: white; 
}
ul .nav a:hover::before{
 top: 0; /* slide tab up */
}

.nav li ul {
  position: absolute;
  display: none;;
  width: inherit;
  margin:0px;
  text-align:center;
  }
  ul li ul li a {
  text-align:center;
 }
 .nav li:hover ul {
   display: block;
  }

  .nav li ul li {
   display: block;  
  }

<div class="nav">
  <ul calss="main_ul">
    <li><a href="#">Home</a></li>
    <li><a href="#">HTML</a>
      <ul>
        <li><a href="#">choice 1</a></li>
        <li><a href="#">choice 2</a></li>
        <li><a href="#">choice 3</a></li>
      </ul>
    </li>
    <li><a class="active" href="#">CSS</a></li>
    <li><a href="#">Javascript</a>
      <ul>
        <li><a href="#">choice 1</a></li>
        <li><a href="#">choice 2</a></li>
        <li><a href="#">choice 3</a></li>
      </ul>
    </li>
    <li><a href="#">jQuery</a></li>
  </ul>
</div>