Using html and css to make a navbar with html code similar to this:

<li><a href="/">Home</a></li></td><td>
<li><a href="/contact">Contact method1</a></li>
<li><a href="/contact2">Contact method2</a></li>
<li><a href="/contact3">Contact method3</a></li>
<li><a href="/contact4">Contact method4</a></li>

What can I add to make the drop down menu stay in front of all other objects, such as embedded videos and a twitter feeds. Currently the drop down will go behind these objects...