It does support HMDs for browsers that have implemented the WebVR spec. That's Firefox Nightly and experimental Chrome builds so far. Without that it makes a best effort at WebVR emulation.

With a Cardboard viewer, A-Frame demos feel pretty consistent with native Cardboard demos to me (tried iOS + Android). But obviously I'd rather try this out with an HMD, and haven't been able to yet. Hoping to pre-order one tomorrow. :)