How can I maintain webplayer aspect ratio on a web page using CSS

I have browsed the CSS forums but nothing that they suggest for maintaining aspect ratios of images and divs seem to work? I want the player to scale fluidly if the user scales the browser window.

Has anyone ever done this? I find it difficult to believe nobody has ever tried using the webplayer on a fluid webpage layout.

Note: My game GUI code works fine, its the proportions of the overall Unity window that I want to maintain. (ie even if the window contained nothing it should still be 4:3)

In my javascript file I have tried using:

 unityObject.embedUnity("unityPlayer", "http://myGame.unity3d", "100%", "100%",params);

but proportions are lost. And if I do a fixed pixel size then obviously the Unity window wont scale to fir the browser window.