This new API is not something added to WinRT, but something that’s available through existing HTML5 APIs. WebGL, the standard for 3D graphics on the web, is available in Internet Explorer 11 as well as WebView controls inside Windows Store apps. However, WebGL is not at this time supported for direct use within apps [this is a correction from my original post] but will be in Windows 8.1 RTM.
WebGL is accessed through the HTML5 canvas object’s getContext method, using the argument “webgl”. The resulting context provides access to the API.
Some talks at //build 2013 that discussed WebGL are New Platform Capabilities for Advancing Web Development (2-067) and Hyper-Fast Web Graphics with WebGL (4-072).