////////////////////////////////

IN CONTENT PAGE TOP

<?php 
$usr_agent=$_SERVER['HTTP_USER_AGENT']; 
if(strpos($usr_agent,"Gecko")){ 
$hr="<div class='div_hr_gecko'><hr class='hr_gecko' /></div>";
}else{ 
$hr="<div class='div_hr_trident'><hr /></div>";
} 
?&gt;

////////////////////////////////

IN CONTENT TEXT

&lt;?php echo $hr; ?&gt;

////////////////////////////////

IN STYLESHEET

div.content hr.hr_gecko{text-align:left;margin:auto;width:50%;}
div.content div.div_hr_gecko{margin:0px;padding:0px;text-align:center;margin-top:2em;margin-bottom:2em;}
div.content div.div_hr_trident{margin:0px;padding:0px;margin-bottom:2em;}

////////////////////////////////