center CSS box on screen

i need to center the CSS alert box on screen in any screen size, possible?


<style>
<!--Message box css-->
div#modalContainer { background-color:transparent; position:absolute; width:100%; height:100%; top:0px; left:0px;  z-index:10000; }
#alertBox { position:absolute; width:300px; min-height:100px; margin-top:50px; margin-left:9em; border:1px solid #000; background-color:#F2F5F6; z-index:12000; }
div#modalContainer > #alertBox { position:fixed; }
#modalContainer #alertBox h1 {color:#fff; margin:0; font-size:0.8em; background-color:#4e9600; border-bottom:1px solid #000; padding:5px; }
#alertBox p { font:0.7em verdana,arial; height:50px; padding-left:5px; margin-left:55px; }
#alertBox #closeBtn { color:#fff; display:block; position:relative; margin:5px auto; padding:3px; border:1px solid #000; width:70px; font-size:0.7em; text-transform:uppercase; text-align:center; background-color:#4e9600; text-decoration:none; }
</style>
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pigmentartsAsked:
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cezarFConnect With a Mentor Commented:
try this....

#alertBox {width:300px; height:200px; position:absolute; top:50%; left:50%; margin:-100px auto auto -150px; border:1px solid #000; background-color:#F2F5F6; z-index:12000; }
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pigmentartsAuthor Commented:
perfect, what did you change? thanks.
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cezarFCommented:
added the following

height:200px;
top:50%;
left:50%;
margin:-100px auto auto -150px;

and removed

min-height:100px;
margin-top:50px;
margin-left:9em;
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