Вариант:
<html>
<head>
<style type="text/css">
.menu {
width: 0px;
height: 20px;
text-align: center;
color: #fff;
background-color: green;
}
.animate {
width: 400px;
transition: width 2s;
}
</style>
</head>
<body>
<script type="text/javascript">
div = document.createElement('div');
function btnClk(){
div.innerHTML = 'Dynamic Menu'
div.className = 'menu';
document.body.appendChild( div );
}
function btnClk2(){
div.className=div.className.replace(/ ?animate/i,'')+' animate';
}
</script>
<input type=button value=Клик onmousedown="btnClk()" onmouseup="btnClk2()">
</body>
</html>