This is one of the core principles of web design. The primary purpose of web design is to enhance the user experience of your visitors. Web design is not entirely a platform for creative expression, instead it is a medium to communicate information. Visitors arriving at your site looking for information should be able to access that information easily. They should be able to navigate the site easily, find the information they are looking for. The design and layout should be easy to understand and information should be readable. Safe size design grids, web safe colors, accessible mark up, fast loading pages, etc. are all components of user centered design.