<div style="position:relative;">
<div id="intro_message">
content
</div>
</div>

#intro_message
{
   position: absolute;
   left: 322px;
   top: 0;
   padding: 20px;
   width: 505px;
}

margin: auto;
position: relative;
width: 505px;

#intro_message
{
   position: absolute;
   left: -137px;
   top: -4px;
   padding: 20px;
   width: 505px;
}

div[c]{
    width: 400px;
    margin: auto;
}

<style type="text/css">
#intro_message
{
   position: absolute;
   left: 30%;
   top: 0;
   padding: 4%;
   width: 505px;
}
</style>

.contains{
position:relative;
margin:0 auto;
width:900px;//or whatever you want
}

<div class="contains">
<div id="intro_message">

#intro_message
{
   position: absolute;
   left: 30%;
   top: 0;
   padding: 20px;
   width: 505px;
}