It's really missing the point by requiring the desktop installation. It's apparently only a "native shell for additional features" but what are those features? You can save to/load from the desktop with data-urls and the file api. I think it's because of some dysfunctional DRM that they're using.
Nonetheless, it's an extremely impressive piece of software. Not sure that CSS3/Transitions is the best tool for this, SVG/SMIL are much more suited for animation. I made a similar tool ~4 years ago (using HTML5, and Ext as well).