JavaScript is mostly the same across platforms, the difference is in the DOM and CSS support. This can be encapsulated in libraries like JQuery.
GWT solves a different (perceived) problem - the lack of classes, inheritance, packages and static type checking in the core language - which is orthogonal to the platform differences.
GWT solves a different (perceived) problem - the lack of classes, inheritance, packages and static type checking in the core language - which is orthogonal to the platform differences.