Each website is a product used daily by people to take actions, not just read the content on it. Your product is amorphous, it takes the shape of whatever container it fills: a mobile browser, a touch enabled desktop browser, or a 30″ iMac that is connected to the Internet via tethering. Photoshop is just one of the means to an end in this new age of utilitarian web sites.
The new technologies available in HTML5 already allow you to create prototypes quickly in the browser. Learn how to create a prototype from start to finish using these new technologies while taking advantage of quick prototyping tools.