The word accessibility refers primarily to the degree in which something is accessible by people with disabilities, but in a wider sense it also measures resistance to external or temporary handicaps, such as noisy environments or bad lighting.

The main method to ensure accessibility is to encode data at as high a level of abstraction as possible, but it is also important to hook into already existing accessibility technologies.

The structure/style dichotomy is a clear example of the former: rather than encoding that something is red, W3C formats allow (and urge) an author to encode first of all the reason why it is red. The redness is added as a rule on top of that. With HTML, for example, rather than <font color="red">, a careful author writes <em class="warning"> and puts the rule that warnings are displayed in red in a style sheet. This way somebody who has no means to display (or see) red at least has a chance to substitute something else that alerts him (such as a nasty sound).