youtube embed code invalid html

http://www.google.com/support/forum/p/youtube/thread?tid=2fe9c0cb02f1558f&hl=en

When going to YouTube, it gives a embed code such as

<iframe title="YouTube video player"
class="youtube-player" type="text/html" width="640" height="385"
src="http://www.youtube.com/embed/QRvVzaQ6i8A?rel=0">
</iframe>

Note that the

type="text/html"

is not valid html. There's no such attribute for iframe tag.

could anyone explain why google put that? I guess it's for some practical reason, but i couldn't guess what.

PS you can get the embed code by going here http://www.youtube.com/watch?v=QRvVzaQ6i8A

i also posted this question on
http://stackoverflow.com/questions/4800227/why-did-youtube-put-a-type-attribute-in-iframe-for-embedded-video
but i hoping some google employee would us some good insight.

Xah Lee