<ul id="navbar">
      <li><a href="#"><big>
            <FONT COLOR=white><B>Меню</B>
          </big></a>
        <ul>
          <li><a href="file:site.html">Главная</a></li>
          <li><a href="file:presentation.html">Презентация</a></li>
          <li><a href="file:code.html">Исходный код</a></li>
        </ul>
      </li>
    </ul>

      #navbar ul{
        opacity:0;
        visibility:hidden;
        background-color: #f90;
        position: absolute;
        top: 100%;
        transition:500ms;
      }
      #navbar li:hover ul { opacity: 1;
visibility:visible}
      #navbar, #navbar ul{
        margin: 0;
        padding: 0;
        list-style-type: none;
      }
      #navbar {
        height: 30px;
        background-color: #666;
        padding-left: 25px;
        min-width: 470px;
      }
      #navbar li {
        float: left;
        position: relative;
        height: 100%;
transition: 0.5s;
      }
      #navbar li a {
        display: block;
        padding: 6px;
        width: 100px;
        color: #fff;
        text-decoration: none;
        text-align: center;
      }
      #navbar ul li { float: none; }
      #navbar li:hover { background-color: #f90; }
      #navbar ul li:hover { background-color: #666; }