UPDATE4: I have created a new version (0.5) with copy to clipboard capabilities, when you click in the embed code input box, code are automatically moved in your clipboard. Since it needs an external SWF file hosted on my web space, I prefere to upload another script with this little modification. You can find the new version here. From this page you could install the old one with have no clipboard copy capabilities (and it doesn't require external script to works :-) ).
NOTE: if you would like to install the modified version with clipboard support, please remove the old version of Youtube HTML valid userscript. UPDATE3: changin the ID and NAME of the input box the code is not selected when you click in the input box, fixed in 0.4.
UPDATE2: in the new version of Youtube, it is excuted a javascript code that update the wrong code re inserting it in input box, to fix (version 0.3) I have changed the ID and NAME of the input box. If somebody have suggestions about an elegant way to do this they are greatly appreciated.
UPDATED: Modified the include domain to works with localized Youtube sites.

Replace the wrong HTML code provided by Youtube to embed a video in you blog/site with a standard valid HTML code recognized by the W3C Validator. I have converted a bookmarklet created by Andrea Micheloni in a Greasemonkey user script:
Andrea Micheloni original bookmarklet

Normal Youtube HTML code

Code provided by Youtube HTML Valid userscript