problem with wp theme

I am designing a new theme for wp and encountered an issue. When I embed a video into the posts or the sidebar the video does not obey the width settings. It automatically stretches to about 1000px (which coincidentally is the width of the page). I can't figure out how to fix that. I figured out that it's likely to be a problem in the css, but I just can't see it.
Can someone help me figure it out?
Thanks.

I figured it out. I had a div class "main" and the video I was trying to embed had an id "main" so it kept resizing itself to the full width.

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That was fast

Thanks.
It was scary as hell though. I made three themes from the same base. Took me about a week. This scared me into thinking that I would have to scrap all the themes and tell people not to download.

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