How can I make a fixed top menubar with a changing height in CSS?

I would like to have a fixed menubar at the top of my website. (which doesn't move when we scroll). My issue is that this menu bar can have a variable height (one or two (or more) levels depending of the number of items/screen size)

Because the top menu is fixed, I have to add a margin-top for my "real" content after that (or it will begin hidden under the menu), but as the menu height is variable, I can't set a margin-top.

So, is it possible to "force" the content not to be under/over my menubar ? How could I do this ? (I wish not to use javascript for this king of positioning)

Here is the code I am using to test a solution :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <meta name="language" content="en" />

    <style>
        body
        {
            margin: 0;
            padding: 0;
            color: #575757;
            font: normal 10pt Arial,Helvetica,sans-serif;
            background: #cdcdcd;
        }

        #page
        {
            margin-top: 0px;
            margin-bottom: 5px;
            width:100%;
            max-width:1500px;
            margin-left:auto;
            margin-right:auto;
        }

        #mainmenu
        {
            background:#b90014 repeat-x left top;
            position:fixed;
            /*height:28px;*/
            width:100%;
            z-index:100;
            left:0;
            background-image:linear-gradient(#b90014, #d00018);
        }

        #mainmenu p#lastUpdate
        {
            float:right;
            color:#ffffff;
            margin-top:5px;
            margin-right:10px;
            margin-bottom:0px;
            margin-left:5px;
        }

        #mainmenu ul
        {
            padding:6px 20px 5px 20px;
            margin:0px;
        }

        #mainmenu ul li
        {
            display: inline;
        }

        #mainmenu ul li a
        {
            color:#ffffff;
            background-color:transparent;
            font-size:12px;
            font-weight:bold;
            text-decoration:none;
            padding:8px 8px 4px;
        }

        #mainmenu ul li a:hover, #mainmenu ul li.active a
        {
            color: #d00018;
            background-color:#FFFFFF;
            text-decoration:none;
        }
    </style>

</head>

<body>

<div id="page">

    <div id="mainmenu">
        <p id="lastUpdate">Last Update : this date</p><ul id="yw0">
            <li><a href="/index.php?r=index">Accueil</a></li>
            <li><a href="/index.php?r=reports">Rapports</a></li>
            <li><a href="/index.php?r=nestedReport">Rapports personnalisés</a></li>
            <li class="active"><a href="/index.php?r=quickReport">Rapports instantannés</a></li>
        </ul>

    </div><!-- mainmenu -->

    <div id="content">
        <div id="contentbox">
            HELLO WORLD
        </div>
    </div><!-- content -->

</div><!-- page -->

</body>
</html>

Thank you !

Solution

I think what you are looking for are media queries:

https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries

You can apply CSS styles depending on the size of the screen.