<div id="menu">
  <ul>
    <li><a href="#">Trang chủ</a></li>
    <li><a href="#">Diễn đàn</a></li>
    <li><a href="#">Tin tức</a>
      <ul class="sub-menu">
        <li><a href="#">WordPress</a></li>
        <li><a href="#">SEO</a></li>
        <li><a href="#">Hosting</a>
        </li>
      </ul>
    </li>
    <li><a href="#">Hỏi đáp</a></li>
    <li><a href="#">Liên hệ</a></li>
  </ul>
</div>
/*==Reset CSS==*/
* {
  margin: 0;
  padding: 0;
}

/*==Style cơ bản cho website==*/
body {
  font-family: sans-serif;
  color: #333;
}

/*==Style cho menu===*/
#menu ul {
  background: #1F568B;
  list-style-type: none;
  text-align: center;
}
#menu li {
  color: #f1f1f1;
  display: inline-block;
  width: 120px;
  height: 40px;
  line-height: 40px;
  margin-left: -5px;
}
#menu a {
  text-decoration: none;
  color: #fff;
  display: block;
}
#menu a:hover {
  background: #F1F1F1;
  color: #333;
}

/*==Dropdown Menu==*/
.sub-menu {
  display: none;
  position: absolute;
}
#menu li {
  position: relative;
}
#menu li:hover .sub-menu {
  display: block;
}
.sub-menu li {
  margin-left: 0 !important;
}

/*==Menu cấp 3==*/
.sub-menu > ul {
  display: none !mportant;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.