Browsers don't have native support for opening WARC. It doesn't solve the safety problem either: you can still construct a WARC that phones home, AFAIK.
It's a great format for the problem it solves, but if browsers supported offline-only files the container format wouldn't (and shouldn't) need to be that complicated.
Im really impressed how snappy it is, i played with it localy a bit cause you never know how the screencast was made etc;
I can confirm this looks just as good as the video, well done