Was this done opening up the standard reply format and NOT the tinyMCE format? I don't have any experience with Safari. I have used FireFox before, these days I am limited to Explorer.
Something I realized with Explorer on my last post is that if I do reply to a post with the tinyMCE, I can paste the entire youtube page (video included) onto the tinyMCE reply page and the playable youtube video also comes up using this method.