Center Authorize.net's Seal Code - I've seen LOTS of solutions out there in the wild to answer this question, and nearly ALL of them are wrong.. This solution works and is syntactically correct.

Center-Authorize-net-Code

<!-- Add this code to your website: Note the only REAL change is on line 6, the addition of a linebreak after the Authorize.net seal --> 

    	<!-- (c) 2005, 2008. Authorize.Net is a registered trademark of CyberSource Corporation --> 
      	  <div class="AuthorizeNetSeal"> 
      	     <script type="text/javascript" language="javascript">var ANS_customer_id="YOUR-AUTHORIZE.NET-CUST-ID";</script> 
	     <script type="text/javascript" language="javascript" src="//verify.authorize.net/anetseal/seal.js"></script>
	     <br />
	     <a href="//www.authorize.net/" id="AuthorizeNetText" target="_blank">Online Payments</a> 
      	  </div> 
    	<!-- end Authorize.Net -->

YOUR_CSS

/* Add the following to your site's stylesheet */
.AuthorizeNetSeal {width:auto !important;} /* required to override Authorize.net seal's CSS and center Authorize.net seal */