Embed with widh relative to height

ive been able to embed my swf as an html object with its height and width relative to the window size, which was step one. now, i want to preserve the aspect ratio, so that the height is always relative to the width. i know this isnt a javascript or html forum but i figured someone here must have wanted to try this at some point. heres what i tried that didnt work:

javascript

window.onresize = function()
{
document.getelementById("containerA").style.width=document.getelementById("containerA").style.height/20+'px';
}

html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
  <head>
    <title>SWFObject - step 1</title>
    <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
    <style type="text/css" media="screen">
    html, body, #containerA { width:100%; height:100%; }
    body { margin:0; padding:0;  overflow:hidden;}
    </style>

    <style type="text/javascript" src="resize.js"></style>

  </head>
  <body>
    <div id="containerA">
      <object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" width="100%" height="100%">
        <param name="movie" value="yankeeScreenMenu.swf" />
        <!--[if !IE]>-->
        <object type="application/x-shockwave-flash" data="yankeeScreenMenu.swf" width="100%" height="100%">
    <param name="quality" value="autohigh" />
    <param name="menu" value="false" /> 
        <!--<![endif]-->
          <p>Alternative content</p>
        <!--[if !IE]>-->
        </object>
        <!--<![endif]-->
      </object>
    </div>
  </body>
</html>