Demo Webconverger with this configuration:

homepage=http://attract.webconverger.com
prefs=http://attract.webconverger.com/webconverger_security_policy/prefs.js
noblank

http://attract.webconverger.com/webconverger_security_policy/prefs.js tells Webconverger to allow us to override fullscreen API restrictions.

Documentation: http://webconverger.org/screensaver/

Update: Since February 2014, Webconverger supports MP4 playback