Is this sustainable and comprehensive? Parts like disallowing '_' at the start or end of a method because some browsers have dangerous methods don't fill me wi

Is this sustainable and comprehensive? Parts like disallowing '_' at the start or end of a method because some browsers have dangerous methods don't fill me with confidence that this is a fool proof solution. It seems like a compiler or iframe would be not only more comprehensive but also wouldn't butcher the language in ways that would make most code need to be adapted.