To play PongVR in a VR headset like the Oculus Rift you need a web browser that supports WebVR. If you're using a Google Cardboard your regular browser will suffice. WebVR is a new technology which enables you to use different headsets to display VR content directly in your browser.
Currently WebVR is only supported by experimental versions of Mozilla Firefox and Google Chrome. Please be aware that WebVR is still in development. For the best VR experience we recommend the WebVR version of Chrome. For WebVR to work the WebVR flag needs to be enabled. Simply go to chrome://flags/#enable-webvr and hit activate.
You can find more information about WebVR on webvr.info.