One of my favorite Mac applications is called Hype, from a company called Tumult. Hype is a GUI tool that builds HTML5 projects, using a timeline approach:
With Hype, you can quickly create web sites, or web apps, and output everything in lovely, standard HTML 5. Drag in some images, set start and end positions, add some events to skip to new sections. You get the idea.
I've been working on some educational apps, and have some prototypes created in Hype, simply because it's the easiest way to create - and easiest way to share - some button-driven multi-page apps with animation, graphics, video and so on. I can quickly create a short segment, and share it with colleagues to watch using their web bowsers, no matter what platform they are using.
It occurred to me that hosting a Hype project in a web control in a Windows 8.1 Modern app might actually work rather well, and so I tried it. Guess what? It does work very well indeed. I was able to create an app in a few minutes that is little more than a web control, and bake the HTML output from Hype right into the Visual Studio solution. Thanks to the powerful IE11 web engine, it ran very smoothly on my little Dell Venue - even some rather sizable video clips played perfectly and started almost instantly. Better than on a Mac in fact!
The latest version of Hype is great at supporting different screen sizes too, and so the "app" runs on a wide ranges of devices.
If you are looking at a quick and easy way to create a prototype, and you own a Mac, I can definitely recommend Hype.