Media Capture and Streams, one of the two primary WebRTC specifications, has just reached Candidate Recommendation stage (W3C announcement here). This is a major milestone and represents the W3C WebRTC Working Group’s belief that the specification is now technically complete and stable enough to implement safely. There will definitely be clarifications added as reports from testing come in, but it is not expected that there will be any major technical changes. It could happen, but the fact of being published as a Candidate Recommendation means the document is very stable now.
This specification is the one that defines MediaStreamTracks and getUserMedia().