CSS To Set Div Always Centered

HTML:

<div id="container_centered"></div>

CSS:

<style type="text/css">
        body{ background-color: cadetblue; }
        #container_centered{
            background-color: floralwhite;
            width: 500px;
            height: 300px;
            position: absolute;
            left: 50%;
            top: 50%;
            margin-left: -250px;
            margin-top: -150px;
        }
</style>

here "margin-left" should be half of the width with (-) symbol and "margin-top" should be half of the height with (-) symbol.

Please give your valuable comments.