I'm a game development expert, been developing games for various platforms for a decade or so. WebRTC is suitable for direct browser to browser data sharing, video streaming, file sharing and whatnot. If you want to implement efficient and real-time multiplayer HTML5 game Node.js is the way to go. It's the most efficient and fastest way of data exchange suitable for all types of multiplayer games, kinda like sockets in C/C++ if you're familiar with that.