I'm thinking this has something to do with Android's support for multiple screen sizes. It turns out that the YouTube content scales correctly on a 10" tablet, but the 7" device I'm testing on is where the problem shows up. Apparently even though you say "399px," it doesn't mean you're going to actually get 399px.
As for the HTML5 inline content, adding:
<metaname="viewport"content="width=device-width, user-scalable=no"/>
to the head seems to solve the problem there.
Still looking for the fix for YouTube. Thinking its going to involve that same meta tag somehow.