While we are working on the new Matroska v3 features, we released new mkclean 0.8.0 and mkvalidator 0.3.3. There are the usual bug fixes and also nicer handling of the Seek Head so there is room for later editing (tags, chapters, etc). The Matroska "semantic" in libmatroska2 is now generated from the same source as the specifications. So there is no possible mismatch anymore (and there were a few ones).
In other news JEBML (our Java library) has been updated so it can handle WebM files as well.