Definitions of Accessibility and Usability

Accessibility is not a clearly defined concept. However, for the purposes of this site, the following definitions will be used:

Definition of Accessibility

A web site or page is accessible if it is reasonably possible for anyone to access the content. As the level of accessibility increases, the differences in ease of access decrease.

Two major causes of difficulty in users accessing content are:

1. Use of a "non-standard" browser, display medium, operating system or browser setup.
2. A disability that makes it harder to view or use the site. This may often be combined with the first cause if the user's disability requires them to use a specific browser or type of browser.

Non-standard browser setup includes obvious things such as whether Java or Javascript is installed and enabled, but also includes less obvious things such as whether the browser over-rides document specified font colours.

Definition of Usability

Usability, like accessibility, is an ill-defined term, and there is a lot of overlap between the two. Again, improvements in usability make it easier to use a site or page, but a usability improvement will make it easier for everyone to view the page.

Overlap between accessibility and usability

There is of course a lot of overlap where an improvement:

• Makes it easier for everyone to use the site or page, and
• Makes it much easier for a certain group to use the site or page.

Example of overlap

The use of stylesheets to describe display and structural markup to describe content improves accessibility by making the document content possible to access in a wider range of display media. However, it also improves usability for everyone by making the documents more easily searchable, and generally smaller in file size, giving a faster download.