With the increasing penetration of mobile devices and tablets, it’s very important for any web based application or website to be built with responsive design principles.

With responsive design, your website will be readable on devices with different display dimensions as it takes care of varying height and width.

WordPress also has a few themes that are based on the responsive web design fundamentals.

Once you have done the infrastructure work to get your web site or application responsive, you can leverage the work across iPhones, Android Phones, iPad, PC, Facebook Application etc., so you can focus your work on the other part of the application instead of rendering.

This is also great for content websites, where the focus usually is getting good content and the design should let the content shine through regardless of the medium that the user is accessing your website from.

This may not be perfect, and you may have to iterate for specific devices, and even put some conditional rules for IE, but certainly better to think about this holistically and before starting the project.

To find more about Responsive Web Design, follow the link to a very good article on this topic. Responsive web design.