The dropdown menus can be easily created with CSS.
Either horizontal or vertical menus are built using unordered list, that is ul. The dropdown feature is created combining the hover pseudoclass along with the visibility property of nested lists.
The following example shows a horizontal menu where the items expand downward.

#dropDownMenu ul li
{
    display: inline; /* remove the list bullets */
    float: left; /* horiziontal menu */
    width: 150px; /* limit the width of every item */
}
#dropDownMenu ul li ul
{
    visibility: hidden; /* submenus are hidden */
}
#dropDownMenu ul li:hover ul
{
    /* submenues become visible when the mouse hovers the parent item */
    visibility: visible;
}
<div id="dropDownMenu">
<ul>
    <li><a href="item1.html">Menu Item One</a>
    <ul>
        <li><a href="item1-1.html">Sub menu item one</a></li>
        <li><a href="item1-2.html">Sub menu item two</a></li>
        <li><a href="item1-3.html">Sub menu item three</a></li>
        <li><a href="item1-4.html">Sub menu item four</a></li>
    </ul>
    </li>
    this is a menu item without a submenu
    <li><a href="item2.html">Menu Item Two</a></li>
    <li><a href="item3.html">Menu Item Three</a>
    <ul>
        <li><a href="item3-1.html">Sub menu item one</a></li>
        <li><a href="item3-2.html">Sub menu item two</a></li>
        <li><a href="item3-3.html">Sub menu item three</a></li>
        <li><a href="item3-4.html">Sub menu item four</a></li>
    </ul>
    </li>
</ul>
</div>

Since in Internet Explorer 6 the hover pseudoclass is supported only by hyperlinks, it is not possible to use CSS dropdown menus.
One solution is to use a conditional directive to expand statically the menu accordingly to the browser version.

[if IE 6]>
<style type="text/css">
#dropDownMenu ul li ul
{
    visibility: visible; /* submenus are always visible */
}
</style>
<![endif]