The relationship between usability and accessibility.

Rami Tabbah - October 27th, 2006, updated June 10th 2009

Usability leads to accessibility and vice versa

The essence of accessibility is to offer comparable access to disabled users.

From developers’ perspective, accessibility at the code level allows the disabled to "access" content and therefore makes it usable. Therefore, accessibility leads to usability.

In our experience, this statement is only partially true and becomes not valid when it comes to complex designs and flash or interactive sites.

Accessibility standards and guidelines ensure basic usability. They do not take into account the user types, their scenarios of use and the task model. Real usability for complex designs can only be achieved using a set of usability guidelines combined with analyzing user's strategies through task analysis. The task model changes with user types and disabled users have different task models. To ensure real usability, we need a separate usability exercise for the disabled users.

Without real usability, we cannot reach real accessibility that achieves comparable access.

Accessibility leads to usability at the low level of the task structure while usability leads to accessibility at the higher level of the task structure.


1- Let us assume that WCAG accessibility guidelines were applied and we got the resulting basic level of usability, which is simply access to all interface elements. Complex designs are still not usable. For example, complex concepts and animations can be replaced by descriptions of the visible content. These descriptions should be written by writers and tested with disabled users. It is not a simple detail the developer will add.

2- The existence of a tab order is not enough. Since the disabled task model is different, a usability exercise is due and the tab order will not necessarily match the layout. This means that there is another hidden design under the visible design and it should be taken seriously and not left to the developer that applies WCAG to the letter. Task analysis and content writing are key at this level.

Accessible design first

Accessibility needs to be addressed at the design level and before development starts Usability specialists, interaction designers and content writers need to design usable and accessible designs and content before the coding phase where WCAG are applied.

If pertinent user centric methods are used, the disabled should be considered categories of users. The normal usability steps apply: personas, tasks, etc.

Failing to have accessibility considered early at the design level and as part of usability planning will lead to complex designs that need to be redesigned to become really accessible and comply with the essence of WCAG.

The secret recipe

What happens most of the time:
1- Build code, ideally follow WCAG at the same time
2- Make it accessible using WCAG, or its essence
3- You now have pages that have components that can be accessed.
If they are simple components, you have a basic level of usability.
If they are complex designs such as conceptual diagrams, the pages are not usable and therefore not accessible because disabled users do not have the same level of access. Other users will be able to understand the concepts.
4- Test for usability and to understand the task model for the disabled.
5- Redesign concepts to simplify them or build an invisible parallel design for the disabled.

What should happen:
1- Integrate accessibility in the usability strategy
2- Design pages that are usable and design content for all types of users including the disabled
3- Create clear requirements for developers
4- Develop using the usability guidelines and WCAG for the code level
5- Test for usability for all types of users

Here the probability of having to redesign is next to nil and the quality of accessibility is superior.