Ensure that the user interface follows principles of accessible design: device-independent access to functionality, keyboard operability, self-voicing, etc.
This is a very short guideline.
When designing a user interface many designers put fashion before function - often at the expense of usability. It is important to remember that even though your design may look good, odds are that it is not very usable. Does this mean good design can't also have good usability? No!
True design will implement simple and usable functions. The tabindex attribute is a nice example of how to implement usability into design.
As a general rule it is better to make web pages more accessible and usable at the expense of design than to build a good looking site at the expense of usability.
In my opinion, Apple shows us that both great design and intuitive usability can be meshed together into awesome product.
Good example of this guideline
-This is a good example because the site uses tabindex for navigation.