Sam Graves

4 Principles of Web Accessibility

The Web Content Accessibility Guidelines (WCAG), the foundation of website accessibility, are organized into four overarching principles: Perceivable, Operable, Understandable, and Robust. 

This blog post will detail these principles, collectively referred to by the acronym POUR, and what they mean in the larger context of web accessibility. 

Each principle covers several guidelines with testable success criteria at Levels A, AA, and AAA.


This principle states that digital content must be able to be understood by multiple senses. 

For example, providing text alternatives for non-text content, such as images, would meet the Perceivable principle because sight isn’t required to understand the content, which benefits users who are blind. 

Another example would be ensuring that color alone is not the only way to convey meaningful information, which helps users with low vision or colorblindness. 

There also must be alternatives to time-based media, such as adding captions and audio descriptions to videos. The Perceivable principle covers Guidelines 1.1-1.4.


The second principle, Operable, states that web functionality must be able to be accessed and used by all users regardless of ability. 

For instance, all functionalities must be available using only a keyboard. This ensures users who cannot use a mouse can access the same information on a webpage that mouse users can. 

Other Operable examples include giving users options to turn off, adjust, or extend time limits on web pages and ensuring digital content does not cause users to have seizures or other dangerous physical reactions that prevent certain users from accessing and using web content. The Operable principle covers Guidelines 2.1-2.5.


The third web accessibility principle is Understandable. This principle says that all content must be able to be understood by users. The operation of user interfaces needs to be understandable to users as well. The first Understandable Guideline, 3.1, states that all text must be readable and understandable. 

For example, all webpages must specify the language of the page and any parts of the page where the language differs from the primary language (for the benefit of screen reader users). Guideline 3.2 says webpages must “appear and operate in predictable ways.” 3.3 focuses on helping users avoid and correct any errors.


The final web accessibility principle covered in the WCAG is Robust. The Robust principle states that digital content must be able to be interpreted by assistive technologies and other user agents. 

The Robust principle covers only one Guideline. It addresses parsing, such as whether HTML elements have complete start and end tags (having complete tags is not critical in many modern browsers that are more “fault-tolerant” of these HTML syntax errors). 

This principle also states that names, roles, etc., of web elements must be able to be programmatically determined by assistive technologies such as screen readers. Status messages need to be programmatically determinable as well.

Importance of POUR

Implementing these four web accessibility principles in your design and development process is the first step to ensuring equitable digital access for all. The principles address users who are blind, deaf, have cognitive disabilities, etc. Because these user-centered principles cover many disabilities, web development teams can be confident that their websites and mobile apps are accessible to all users. 

The guidelines and especially the testable success criteria get much more specific and technical, but these four general principles provide a good high-level understanding of website accessibility and can be understood by anyone regardless