Sharepoint Designer Rendering

Hi,

Just run up Sharepoint Designer and pointed it at a website I'm working on (non Sharepoint) to checkout some CSS stuff.

Unfortunately Sharepoint Designer doesn't render the page in the same way as IE7, (or IE6, FireFox for that matter).

Is there some simple setting I've missed in Sharepoint Designer that makes it render properly

I like the indication it gives of what style is relevant to particular elements, but it is of limited value if the rendering is not right.

Thanks

Martin