html - how to horizontally center a responsive div? -

trying center div contain responsive adsense code.

example :

<div class="wrapper">     <div class="adsense">adsense code here</div> </div> 

i can center div if type exact measurement of ad. 728x90 max want. @ moment have img temp placement , works fine. problem if type in exact px wont responsive.

any ideas please?

ok actual code...

<section class="main-content">             <div class="top-banner">                 <img src="img/ad_top.jpg">             </div> </section> 

(img there placeholder notepad++)

.main-content {     float: left;     width: 75%; }  .top-banner {     margin:0 auto; } 

this places img left , not center.

what talking about? horizontal centering done margin: 0 auto; doesn't matter size container has.