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The Four Principles of Accessibility

To help simplify the complex subject of accessibility, we’ve put together a quick guide to accessibility’s P.O.U.R principles.


Accessibility is a broad topic that ranges from abstract legal rulings to specific coding guidelines. The industry standard guidelines for web content accessibility are organized around four principles: Perceivable, Operable, Understandable, and Robust (or POUR). To help you understand the basics of accessibility, we’ve put together an introduction to these principles.


Starting at the most basic level, users must be able to process information. Information that is not presented in a processable format is not accessible. Among other affordances, this means providing text for those who cannot hear, and audio for those who cannot see. It does not mean creating audio for all text, but content must be consumable by screen readers and other assistive technologies. Websites and apps that require sight or hearing won’t pass the test of perceivability.

Ask yourself: Is there anything on our website that a blind, deaf, low vision or color blind user would not be able to perceive?


People with disabilities need to be able to operate websites and applications with a variety of tools. Many users with disabilities cannot operate a mouse. Alternatives like keyboard-based operation should be implemented.

To help users with cognitive disabilities operate a website, animations and media should be controllable, and time limits for completing an action should be generous or configurable. Most importantly, sites and apps should be forgiving. All people, not just those with disabilities, make mistakes. Offer second chances, instructions, cancellation options, and warnings to help all users.

Ask yourself: Can all functions of our website be performed with a keyboard? Can users control the interactive elements of our website? Does our website make completing tasks easy?


If users can perceive and operate a website, that doesn’t mean they can understand it. Understandable websites use clear, concise language and offer functionality that is easy to comprehend. If a user takes an action, the connection between the action and the result should be obvious. Navigation should be used consistently across a site. Forms should follow a logical flow and provide clear labels. If a user must go through a process — like a checkout — adequate guidance should be provided. If this feels like usability and not accessibility, that’s because usable websites are inherently more accessible.

Ask yourself: Is all of the text on our website clearly written? Are all of the interactions easy to understand?


Users pick their own mix of technologies. Within limits, websites should work well-enough across platforms, browsers, and devices to account for personal choice and user need. While users cannot expect a website to support Internet Explorer 1.0, sites should not dictate the technology users can use. When sites dictate supported technology platforms, they restrict access for any non-conforming user. One of the best ways to meet the principle of robustness is to follow development standards and conventions. Clean code is generally more robust and consumable across platforms.

Ask yourself: Does our website only support the newest browsers or operating systems? Is our website developed with best practices?

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WillowTree Staff

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