Often, when designing web applications, I skip Photoshop and jump straight into CSS3 and HTML5. Now that there are these powerful technologies, designing static PSD mockups seems a bit redundant. Clients also tend to appreciate this because they can interact with an HTML prototype of the product, getting a much better feel for the user experience.
“But what about older browsers?” I hear you cry! Surely CSS3 isn’t ready for prime time yet? Well, the answer to that is graceful degradation. Older browsers will ignore your CSS3 styles, falling back to the standard ones. For example, in Chrome, your users will see the application in its full glory, gradients and all, whereas in IE7, the application will be just as functional, only a lot less pretty.
As for Internet Explorer 6, I advocate you to drop support altogether. Facebook, Amazon, and Google are all starting to drop support, and the small percentage of IE6 users just doesn’t make the effort to support it viable. The Web is moving on, and older technologies need to be dropped.