There has been recently a huge discussion on designing in the browser vs. doing it in Photoshop/Fireworks. While checking out your mockup in the browser is great, this mechanism it’s not suitable for all sites.
Turns out that as web designers, we design interfaces. You can think of them as part of a complex set of interactions: between your site and your user. Most people will agree in naming interfaces and interactions components of a science called User Experience Design.