== Open Spec? == 

 Article says that it is an open specification, which I believe is incorrect.  Microsoft published some tools for authoring HDi, and maintains a forum and a blog concerning it.  The only way to get the complete specification, however, is to sign a NDA and pay $5000 to the DVD Forum.  The open parts are css, Javascript, and SMIL, which are all approved by w3c.  The important parts, though, such as Javascript methods and xml syntax, are only available through the spec that requires an NDA.