Play HTML 5 tags via KDE Phonon + storing passwords in Kwallet (like gnome-keyring support in webkit-gtk)
Sorry, but this is not a very useful enhancement request.
First of all, there is support for phonon based playback of HTML 5 audio/video elements in trunk.
In addition the support for kwallet is a feature that may not belong into WebKit itself but a browser might be a better location. Arora for example implements support for password storage on top of QtWebKit.