The Basic Principles of Design
The basic principles of design are true for Web design as well as other design. They show you how to put together design elements in an effective manner. Design is more than just slapping HTML tags up onto a page, and using these principles will help you build more pleasing and useful designs.
Balance is the distribution of heavy and light elements on the page. The principle of balance shows you how to lay out your pages so that they work. Larger, darker elements appear heavier in the design than smaller, lighter elements.
When most people think of contrast, they typically think of colors or black and white. But there is more to contrast than color. You can have contrasting sizes (large vs. small), or contrasting textures (smooth vs. rough), or contrasting shapes (square vs. circle).
Emphasis is what the eye is drawn to in a design. It's tempting to emphasize everything in a design or try to give everything equal emphasis, but this ends up making the design bland and flat. Instead, as a web designer you should define the hierarchy of the web page and then apply the emphasis to the elements based on that hierarchy.
Rhythm brings an internal consistency to your Web designs. Rhythm is also called repetition. Patterns are easy for humans to comprehend, and repetition provides patterns that make your website easier to comprehend.
Unity is also called proximity. It is the principle of keeping like elements together and diverse elements further apart. Unity pulls elements together.