The Web Platform Working Group has published a Proposed Recommendation of WebIDL Level 1. This document defines an interface definition language, Web IDL, that can be used to describe interfaces that are intended to be implemented in web browsers. Web IDL is an IDL variant with a number of features that allow the behavior of common script objects in the web platform to be specified more readily. How interfaces described with Web IDL correspond to constructs within ECMAScript execution environments is also detailed in this document. It is expected that newly published specifications reference this document to ensure conforming implementations of interfaces are interoperable. Comments are welcome through 14 October.