"Added to HTML5" is misleading. More specifically, the bitcoin scheme was added as a valid scheme that websites may register themselves as handlers for via navigator.registerProtocolHandler, a list that already includes mms: xmpp: webcal: tel: and other misc schemes. It doesn't specify any kind of support for the scheme for the browser.
See http://www.whatwg.org/specs/web-apps/current-work/#whitelist...