Good question.
HTML5 GM will be more like SilverCreator. Syntax wise and structure.
Javascript has a very free structure. HTML5 GM will have a set structure to make it easier. Plus, dare I say it, syntax highlighting!
Right now I'm coding it in TextEdit and haven't found a nice, simple, light weight javascript editor that works as well as TextEdit.
Yeah, all commands will be in the docs and this will be tailored specifically to game making with the HTML5 canvas. Where as Javascript is open to everything, this will be narrow.
The HTML5 GM will be way easier than making a game in Javascript.