Sorry if this is a repost (didn't see my message from reportbug show up).
I'm glad to hear this package will start using gstreamer1.0
But, currently in Sid, libqt5webkit5 is not fully functional as it cannot play h.264 encoded video (ie: youtube) because gstreamer0.10-ffmpeg is not installable.
I hope qtwebkit5.5 and gstreamer1.0-libav fixes this, or that some other solution is found soon.
Please see this bug report of an affected project:
Much thanks,
Elliot