There are several tools out there to create HTML5 Banners. So far the problem with those tools are that they are very tied to the platform you need to design banners for.
I recently used Flashtalking Ad Builder to create HTML5 Banners (Ad Builder documentation). The tool is very flexible and the output is very close to Flash results however all the code exported is a JSON file that describes the animation (almost 32k in size).
With Google Web Designer GWD I've have learnt a trick or two since you can explore the resulting code. There are other tools out there like TweenUI which is basically an interface for GSAP and BannerFlow and HTML5Maker which are services on the cloud.
Is my opinion that a winner has not yet arise because most of the tools try to be very much like Flash, however HTML5 although similar is different enough so any code done with a timeline UI is not very usable in a code environment, therefore creating resizes or slightly different versions is very difficult.
As a developer you should still be comfortable writing your own code and creating your own workflows to achieve the client desired result. If you plan to invest in software let it be a Sublime license and whatever training in CSS/JS Animations you can get your hands on.