PS. https://Clara.io also has support for physically-based rendering in WebGL, just use the "Physical" material. :)
It is hard to compete with Unity with the same efficiency on all the non-web platforms platforms supported by Unity when it has to run in a browser/JavaScript. But for web-based games PlayCanvas beats out Unity clearly.
For the moment at least, if you discount the Unity plugin approach.
Unity 5, the coming version, features a (plugin-less) WebGL-export (and I believe the Unity 5 beta already supports it):
https://blog.mozilla.org/blog/2014/03/18/mozilla-and-unity-b...
Also, by physically-based rendering I assume they mean physically-plausible shaders.
BTW one of the best references is this: http://blog.selfshadow.com/publications/s2013-shading-course...
So I opened it in chrome and it hangs my iMac. I do graphics programming and use 3D a lot, and it is the first time my machine hangs.
