Internet Explorer has a proprietary extension known as conditional comments. These are specially formatted comment tags that let you include content based on the browser version. I generally dislike proprietary extensions, but this one I am a fan of. It respects the fact that browsers have rendering differences and designers will need to adjust their content. It does so without requiring javascript to be enabled.

<!--[if IE]>
<!--[if !IE]-->

This doesn't work. In fact, in IE, these aren't even considered comments.

<!--[if IE]><![if !IE]><![endif]-->

You can see this content if you are not in Internet explorer.

<!--[if IE]><![endif]><![endif]-->

This means that to insert quicktime into a page in a way that is standards compliant, you can do:

<object classid="clsid:02BF25D5-8C17-4B23-BC80-D3488ABDDC6B"
        width="240" height="188">
 <param name="src" value="../sites/default/" />
 <param name="controller" value="true" />
 <param name="autoplay" value="false" />
 <!--[if lte IE 6]><![if !IE]><![endif]-->
 <object type="video/quicktime"
         width="240" height="188" class="mov">
  <param name="controller" value="true" />
  <param name="autoplay" value="false" />
   <p>Error: Quicktime not available.</p>
 <!--[if lte IE 6]><![endif]><![endif]-->

This should parse as valid XHTML, and it allows the proper support of the object tag that is slated for IE7.