When moving one container down both move
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So I have a container called banner within a container called main as so:

    <div id="main"> <!-- BEGIN MAIN -->
    <div id="banner"> <!-- BEGIN BANNER -->
    </div> <!-- END BANNER -->
    </div> <!-- END MAIN -->

Main is a gradient and banner is the banner I want to have kind of in the middle of that gradient. Left and right auto works great. But when I try to put a margin of 10px on top of the banner, both the main and banner move down. How do I make it so only the banner moves down? Thanks a bunch!

    #main {

	width: 1200px;
	height: 440px;
	background-image: url(../website/images/background_bg.png);
	
	}
	
    #banner {

	width: 966px;
	height: 367px;
	margin-left: auto;
	margin-right: auto;
	margin-top: 10px;
	background-image: url(../website/images/banner.png);
	
	}