Very exciting. This is a great way to get people playing your game without worrying about plugins. It will be interesting to compare the weight of a basic 2D game using Unity 4.3 2D engine to one written in a HTML5 framework such as Impact. I'm sure the use case is more for 3D gaming on WebGL, but using one toolkit for all development is definitely easier for the developer.

Regardless, this is great news and great trend. Corona SDK also announced WebGL support is coming soon [http://coronalabs.com/blog/2014/03/07/coronacards-is-cool-he...]. Lots of great new tools to play with.