Content must be robust enough to used by a variety of technologies and assistive technologies

The four principles of accessibility lay a foundation for anyone to access and use online content, and does not prohibit or restrict access to information for people with disabilities. The four principles are perceivable, operable, understandable, and robust.

Online content should be Perceivable or available to be accessed by all users. An example of perceivable content is properly using alternative text (alt text) for images. Alt text is a line of text that accompanies an image to describe the purpose of the image, so that someone using a screen reader will understand all intended information of the image.

Operable means the user interface can’t require a user to perform a task they can’t complete. An example of operable content is not to use pop ups that require verification methods, such as CAPTCHAS that limit users with disabilities from accessing content. CAPTCHAs (Completely Automated Public Turing Test to Tell Computer and Humans Apart) are designed to prevent robots and automated systems from filling out forms and can include distorted text and audio, as well as cause a number of issues with access to the intended information.Information is Understandable, where language and navigation is consistent and not descriptive.

Information that is Understandable refers to keeping language and navigation elements consistent, and keeps reading order logical and meaningful. To do this make sure you give accurate descriptions on where to find and access information, and maintain consistency on the placement of home buttons. For example, if you create a home button at the bottom center of a page, create the home button in the same place on all additional pages. An example of consistent language throughout your course may be: “Go to assignment one to begin the next step. You can also find assignment one under the assignments folder”. A common mistake may be: “Click on the green arrow at the top of the screen to go to the first assignment”. This final instruction names the element by its color and shape and provides descriptions that will be unknown to a user with visual impairments. By using descriptions that adequately name objects, you are making it easy to locate information for all users, not just those with a disability.

Content should be Robust enough that it can be read by a variety of technologies and assistive technologies. Some things to remember when designing an online course, site, page, or module is that a mouse or trackpad is an inaccessible object. People who are blind or with a visual impairment can’t see where to click the mouse or someone may have a tremor or other disability proving difficult to click or navigate using one of these technologies. Ensure your course can be accessed through use of keyboard navigation (arrow keys or keyboard shortcuts) and remember to include descriptive and accurate language when directing a user to a certain part of the site. Someone using a screen reader may use the tab key on a keyboard to navigate through a page, press certain keys to perform functions such as bring up all the links on a page, or display all structural lists or bullets. Another example of robust content is to build in heading structures so content can be easily navigated by a screen reader or any new developments in assistive technologies.

Introduction to Understanding WCAG 2.0