Thanks UW... it was your post about MovieRoom's new Smart banners that raised this question for me.
The Flash ads will actually sit inside sidebars in the real layout, so it's not really an exact 33% size that I'm after. I think I posted too simple an example page. What I'm trying to do is have the Flash ads resize to 100% of the enclosing container, in this case a division tag.
I'm addressing both 'embed' and 'object' in my stylesheet. What I did is make a div with an id="flash_ad" to hold the content and then set the embed and object selectors in that div to 100% width in my stylesheet. It does give the illusion of resizing with some Flash ads, like the MovieRoom ones, and it works a little better with embedded Flash videos from some other sponsors. Not dynamic resizing, but resizing on window reload at least.
Here's the new test page with a few examples of what works and doesn't. I found that setting height to 100% in this case is a bad idea, as you can see.
http://cashfetish.com/smn/tests/resizing-tests2.html
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