This is a great tutorial on all the four basic design principals. The creator has made remember the principals of design easy by calling it the C.R.A.P design principals. C.R.A.P standis for contrast, repetition, alignment and proximity.
This is a great site for information on the principle of contrast because it organizes the methods of color contrast, size contrast, and alignment contrast. It includes examples for each and is extremely easy to read and comprehend.
A web design is made of many different elements, each having varying levels of importance and some demanding prominence over others. << more detailed information on the importance of contrast.
Chapter 6, Repetition: An analysis of contrast, repetition, alignment, and proximity with examples of each in the form of website design. This shows how they all work in harmony and how they impact the design element of a website.