From what I understand, the HTML+Javascript WinRT APIs are really only supported on the "full" Windows RT/Metro environment, not Windows Phone 8. Windows Phone 8 supports C#/VB+XAML, C/C++, and you can use HTML and Javascript, but only in a web browser control, which is not the same as a true Javascript API with hooks into lower-level constructs. Apparently, you can also run legacy Windows Phone 7 Silverlight apps on Windows Phone 8 as well, but that is just a stopgap to not completely cut ties with the legacy apps.