WCAG principles

WCAG (Web Content Accessibility Guidelines) is organised around four principles: perceivable, operable, understandable and robust. Each principle has guidelines describing what accessible web content should do, and a site needs to meet all four, not just one.

Principle	What it means	Guidelines in practice
Perceivable	Users must be able to receive the information.	Provide alternatives for time-based media, such as captions on a recording of a school concert; present content in different ways without losing information or structure; separate foreground from background so text is easy to see and hear.
Operable	Users must be able to interact with the interface in a meaningful way.	Make all functionality available from a keyboard; give users enough time to read and use content; avoid content known to cause seizures, such as flashing images; help users navigate, find content and work out where they are.
Understandable	The information and the operation of the interface must be understandable.	Use text that is readable and understandable; make pages appear and operate in predictable ways; help users avoid mistakes and correct them.
Robust	Content must keep working across a range of devices and with the assistive technology people use to access it.	Build pages so they keep working with current and future user agents, including browsers and assistive technology.

For example, a school library search page is perceivable if its labels stand out clearly from the background, operable if the search box and buttons can be used with Tab and Enter, understandable if the instructions are clear and any mistakes are explained, and robust if it still works with a screen reader on a tablet. The principles work together: a page that is easy to see but impossible to use without a mouse is not accessible.

Practise this