I wonder whether this supports all the simple animation features that made flash big back in the day (like vector animation, tweening, etc.). When people say HTML5+js is the replacement for Flash, they usually think about video players and ActionScript apps.
I also wonder, if you go through all the trouble of creating a new flash implementation, why do it in HTML+js? Why not in native code on top of a cross-platform library, such as Skia or Qt? Security is one issue, but if you start a new project you can plan for security from the start and effectively rule out buffer overflows etc. with the right programming practices. Also all modern OSes have ways to sandbox applications.
I'd very much rather have my code in small, auditable, removable plugins than put everything and the kitchen sink in the browser. Sure, you prevent buffer overflow exploits by using a well tested JS engine, but you get a load of other possible bugs that you wouldn't have if the plugin were in a separate process (DoS/hanging the browser, information leakage, cross origin scripting, etc.).