I’ve just run the front page of the blog through the W3C validator, and discovered that although my own code is cool, the page won’t validate as long as there’s a YouTube embedded movie on here, because “embed” is not, in the view of the W3C, an element, even though it’s now supported in most browsers. It happens that this post will actually knock the last post of that nature off the front page, which I’m confident will be valid again. Yo.
It’s late and I’m tired, but maybe tomorrow I’ll look further into this. I expect that if the YouTube code is invalid, someone must have noticed, like a year ago. There would have been some outcry, somewhere.
It strikes me that the smart-alecky thing to do now – assuming for the moment that it isn’t possible to embed YouTubes with valid code, which I doubt, actually – would be to wrap the Valid buttons (in the meta links section of the sidebar) in a script which displays them providing there is no YouTube present… understanding that when I said “now” just then, I didn’t mean now…
(UPDATE: as I suspected, the problem has been solved already. Replace YouTube’s embed spew with either:

<object type="application/x-shockwave-flash" style="width:425px; height:350px;" data="http://www.youtube.com/v/n"><param name="movie" value="http://www.youtube.com/v/n" /></object>

or:

<object type="application/x-shockwave-flash" width="425" height="350" data="http://www.youtube.com/v/n"><param name="movie" value="http://www.youtube.com/v/n" /></object>

where n is the ID number of the relevant YouTube video. Thanks to this post and comments on Bernie Zimmerman’s blog. Now, what would it cost YouTube to amend their scripts to produce valid code in the first place?)