The problem in my instance (with WordPress) is, I believe, two-fold.

First, WordPress itself tries to re-form HTML to “correct” errors: that messes up the poorly structured (but generally functional) default HTML provided by Youtube. It also can mess up some correctly formed HTML that WordPress doesn’t understand.

Second, browsers that more strictly interpret HTML correctness (E.G.: IE 7) may reject incorrectly formed HTML even if I manage to get WordPress to stop messing with things.

So, the answer (in my case) was to re-form the HTML into something that is correctly formed and to get get WordPress to stop “updating” the HTML by smacking it upside the head by disabling it’s rich editor. I don’t know what blog software you are using, Oblivions, but try my second example above and see if it works better.