It's no different than on the web. You don't write HTML with Javascript for the web and you won't do it in WinRT.

>You mean using JavaScript to write HTML5? I sure don't want to develop anything with a UI using nothing but JS. I must be misunderstanding you.
>
>I know HTML5 is not all the way through the standards process yet. But it is already being used in 30-something of the busiest web sites.