ul { /* all lists */
    padding: 0;
    margin: 0;
    list-style: none;
}

li { /* all list items */
    float: left;
    position: relative;
    width: 10em;
}

li ul { /* second-level lists */
    display: none;
    position: absolute;
    top: 1em;
    left: 0;
}

li>ul { /* to override top and left in browsers other than IE, which will position to the top right of the containing li, rather than bottom left */
    top: auto;
    left: auto;
}

li:hover ul, li.over ul { /* lists nested under hovered list items */
    display: block;
}

2 level dropdown menu

CSS