Zia.js is an XNA-inspired, object-oriented, WebGL-based game framework. Right now it’s focused on graphics. Sound, networking and general-purpose game APIs will be added later (if there’s enough interest).
Zia.js is not a graphics engine, and it’s not a game engine. It could help with building either of those, but it’s not one of those. It is a framework that can be used to build a game, or a game engine.
Just like XNA gave game developers a friendly set of APIs to use to build games on Windows-based platforms, I’m hoping to do the same for the web with Zia.js.