The main thing is performance, which html5test doesn't measure - most of those points come from document features for web sites.
Still - iOS 6 has the web audio API, which if it works, is totally awesome! No more one-sound-at-a-time in Safari!
Yes. I noticed new updates may improve the performance, i'm not sure if they say is true.
The good thing is about iOS 6 implemented more HTML5 elements.