Understanding accessibility principles - Perceivable

There are four principles in the Web Content Accessibility Guidelines (WCAG) which lay the foundation for anyone accessing and using web content. In this edition we provide specific detail and examples about the first principle, Perceivable: Information and user interface components must be presentable to users in ways they can perceive.

Text alternatives for non-text content

Provide text alternatives for any non-text content so that it can be changed into other forms people need, such as large print, braille, speech, symbols or simpler language.

Text alternatives are equivalents for non-text content. Examples include:

  • Short equivalents for images, including icons, buttons, and graphics
  • Description of data represented on charts, diagrams, and illustrations
  • Brief descriptions of non-text content such as audio and video files
  • Labels for form controls, input, and other user interface components

Text alternatives convey the purpose of an image or function to provide an equivalent user experience. For instance, an appropriate text alternative for a search button would be "search" rather than "magnifying lens".

Text alternatives can be presented in a variety of ways. For instance, they can be read aloud for people who cannot see the screen and for people with reading difficulties, enlarged to custom text-sizes, or displayed on braille devices. Text alternatives serve as labels for controls and functionality to aid keyboard navigation and navigation by voice recognition (speech input). They also serve as labels to identify audio, video, and files in other formats, as well as applications that are embedded as part of a website.

Captions and other alternatives for multimedia

People who cannot hear audio or see video need alternatives. Examples of alternatives for audio and video include:

  • Text transcripts and captions of audio content, such as recordings of people speaking
  • Audio descriptions, which are narrations to describe important visual details in a video
  • Sign language interpretation of audio content, including relevant auditory experiences

Well-written text transcripts containing the correct sequence of any auditory or visual information provide a basic level of accessibility and facilitate the production of captions and audio descriptions.

Content can be presented in different ways

Create content that can be presented in different ways (for example simpler layout) without losing information or structure.

In order for users to be able to change the presentation of content, it is necessary that:

  • Headings, lists, tables, and other structures in the content are marked-up properly
  • Sequences of information or instructions are independent of any presentation
  • Browsers and assistive technologies provide settings to customise the presentation

Meeting this requirement allows content to be correctly read aloud, enlarged, or adapted to meet the needs and preferences of the user. For instance, it can be presented using custom color combinations, text size, or other styling to facilitate reading. This requirement also facilitates other forms of adaptation, including automatic generation of page outlines and summaries to help users get an overview and to focus on particular parts more easily.

Content is easier to see and hear

Distinguishable content is easier to see and hear. This includes:

  • Color is not used as the only way of conveying information or identifying content
  • Default foreground and background color combinations provide sufficient contrast
  • Text is resizable up to 200% without losing information, using a standard browser
  • Images of text are resizable, replaced with actual text, or avoided where possible
  • Users can pause, stop, or adjust the volume of audio that is played on a website
  • Background audio is low, or can be turned off, to avoid interference or distraction

Meeting this requirement helps separate foreground from background, to make important information more distinguishable. This includes considerations for people who do not use assistive technologies and for people using assistive technologies who may observe interference from prominent audio or visual content in the background. For instance, many people with color blindness do not use any particular tools and rely on proper design that provides sufficient color contrast between text and its surrounding background. For others, audio that is automatically played could interfere with text-to-speech or with assistive listening devices (ALDs).

Distinguishable: Make it easier for users to see and hear content including separating foreground from background.

Content reproduced from World Wide Web Consortium (W3C) website