Nic wrote Long story short: the onmouseover and onmouseout events are terrible. Which is why at this point people generally switch to jQuery and use the respective jQuery functions. Especially because you want mouse events to work no matter what, as they are the most important events.

The onmouseover and onmouseout events fire every time you go over/leave the menu, but unfortunately also for all child elements, which explains the flickering. Remove the child elements (<ul>, <li> and <a>) and it will work just fine. But that would make for a lousy menu.

Because you are not allowed to use jQuery, I suppose you need to search the web for a JavaScript snippet that will properly handle the 2 JavaScript events, but I can imagine it's going to be more code than you might be comfortable with.

Alternatively, whoever said you cannot use jQuery, turn on smart-ass mode and tell them you don't need JavaScript at all, and simply achieve a similar effect with nothing but CSS3.

Click to View Content

HTML


<div id="menu">
    <ul>
        <li><a href="#">Menu</a></li>
        <li><a href="#">Order</a></li>
        <li><a href="#">Contact</a></li>
    </ul>
</div>


CSS


#menu {
    background-color: #c9f5ff;
    width: 100px;
    height: 30px;
    line-height: 30px;
    text-align: center;
    overflow: hidden;
    -webkit-transition: height 250ms;
    -moz-transition: height 250ms;
    -o-transition: height 250ms;
    transition: height 250ms;
}

#menu:hover {
    height: 90px;
}

#menu ul {
    margin: 0 auto;
    padding: 0;
    list-style: none;
}

#menu a {
    text-decoration: none;
}


It's not the most ideal as it requires you to update CSS depending on the content, but it might just do depending on your assignment. Make sure the "height" and "line-height" properties for "#menu" are equal at all times, and the hover's "height" property should be the menu's "line-height" multiplied by the amount of items in your menu. Also, Google for the CSS transition property for more info on what's possible, my code is nothing but basics.