In a recent interview I faced the same question, but at that time I was not aware of the term, so I thought, may be some people like me are also in this world, and my findings will help then to expand their knowledge base.
After spending 7 long years in the web design industry, I may be very aware of the individual terms but not very local with the acronym for a design principle. CRAP is actually an acronym for a design principle that was first coined by author Robin Williams.
So what is it?
Contrast is the differences that make an object distinguishable from the background and any other objects. That means that your graphs, illustrations and anything else that you want to stand out should be different or contrast with the print to make them more noticeable. Elements that are not supposed to be communicated in the same light should be different because making things slightly different might confuse the viewer into thinking there might be a relation between two things when the relationship really does not exist. Contrast in your design livens things up and makes things easier for people to understand.
If there is a relation between two points, the style should repeat for a consistent feel. Repetition means keeping design for similar themes consistent. Many people often skim text and graphics, repetition can help communicate your points when people only want to skim the message that you want to communicate. It reinforces communication and ultimately helps the reader understand. Just remember to repeat things in a consistent manner and don’t overdo it. You want to make your repetition readable and effective.
You need to align things in an organized fashion. Everything needs to visually connect with something else that the viewer is looking at. This means that your headings and subheadings should follow the same alignment and you don’t want things placed where viewers will not look. Nothing should be out of place or disconnected, if it is then it probably doesn’t belong.
Proximity is used to create relative meaning in your web design. Related elements should be grouped together; while separate design elements should have enough spacing between them to communicate they are different. You should use your document’s white space to separate key points. Everything should look professional.
Good design is always CRAP, and it is always a handy design principle. Remember to check contrast, repetition, alignment and proximity before you publish any of your content and designs. Ensure that your web design and any other media you produce is effective by double-checking your work for these design principles before publishing, it will really pay off in the long run.