WebIDL checker
A growing number of W3C groups are in the process of developing JavaScript APIs: the HTML Working Group, the Web Applications Working Group, the SVG Working Group, the Geolocation Working Group, and the newly created Device APIs and Policy Working Group of which I am one of the Staff Contacts.
The Web Applications Working Group has been developing a common IDL format to define these APIs, called WebIDL, which basically takes the OMG IDL format and extend it to make it more useful and informative in the context of JavaScript APIs.