Even if you somehow manage to fully implement all of JS with decent performance, you will finally hit the wall with video DRM - your users will never be able to watch Netflix, for example.
Widevine is the first DRM to be really open to be ported. They suck the least of all DRM systems. This is the DRM used in the Chrome browser and Firefox. I think it would be possible (not easy, or logical, but possible) that Serenity Browser gets Widevine at some point.
