Hi,
When we add a flash file using the flash button, it does not create a valid xhtml.
Example with my website, the created code is :
<embed height="518" width="691" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" src="/sites/mindenice.fr/files/articles/la.redaction/2008/2008-10/cellopack.swf" play="true" loop="true" menu="true"></embed>