The Open API Initiative

I'm not sure about WSDL, the only thing I understand about it is that it was originally viewed as a simplified version of CORBA for Web :) But did XML Schema actually had that cycle of incompatible extensions you describe? I agree it's a bit heavyweight, but otherwise its concept is pretty clear and solid. To me HTML and CSS look more like vendor extension soup you describe.

That said, there certainly will be extensions, because REST as it is is not sufficient for many scenarios. Take WebDAV, for instance: it has locks, batch updates, queries, standard support for sync, etc. REST has nothing of this sort. Have you ever tried to sync a large dataset against a typical REST API? It's very painful. While REST is simple, this simplicity mostly comes from being very basic. And WSDL and XML Schema are complex not because their authors are stupid (addition: or because there was some political wars), but because the subject itself is complex.

Too little, too late.

Thrift, gRPC, YaRPC, etc. are all RPC frameworks with strong typing and binary transports. They are still immature, but as such frameworks mature and support more languages, they will eat the lunch of any project trying to build on REST.