According to the "Your App, Your Website, and Safari" WWDC session from this year [1] (Slide titled "Signing up with confirmation" towards the end of the document), you simply use the `current-password` or `new-password` autocomplete attribute again for confirmation of either field.

[1] (Session PDF Link): http://devstreaming.apple.com/videos/wwdc/2014/506xxeo80e5ky...