WCAG 2.0 Part Two

Perceivable:
The intent of the perceivable principle is to make the website more friendly to those who might not be able to see, have bad vision or colourblind.

To counteract this, a good example is using alternative text for images, so that a Screen reader can read the image as plain text and describe it, if the person is blind or can't see well.

Operable:
The intent of operable is too much sure no one is unable to view the site due to bad design choices, such as using flashing lights that can cause seizures for some.

Another good operable design choice is allowing navigation to be possible solely by using the keyboard, however, this would mean no components for the website such as Flash or Java because they disable keyboard use for the webpage JavaScript when in focus.

Understandable:
The intent of the understandable principle is to make sure that the webpage adheres to predictable layout and text that is easy to interpret and read.

For example, any text that is abbreviated and used in an unusual fashion should be able to be interpretted and read properly by a Screen Reader or someone with bad eyesight.

Another thing that will make the website easier to understand is little conventions such as the navigation bar being on the left or top. Content being placed on the right or below the navigation, etc.

Robust:
The intent on making a site robust is so that it renders everything properly and as fast as possible.

To achieve this, you need to make sure all your HTML tags start and end, none of the ID tags stay the same and elements do not duplicate attributes.