What Is Perceivability in Web Accessibility? WCAG Principles Explained

The Web Content Accessibility Guidelines (WCAG) are the leading international standards for accessible web design. Understanding how these guidelines are organized can make them easier to implement, and luckily, they have a very clear system of organization: Each of the standards contributes to one of four overarching principles. According to WCAG, accessible website design should be Perceivable, Operable, Understandable, and Robust for all users — including the 25% of US adults who have disabilities. 

But what do these terms mean? In this series on the WCAG “POUR” principles, we’ll introduce each of the four major tenets of accessible web design in turn—starting with this post exploring what WCAG means by “perceivable.”  Here’s a quick introduction to perceivability in the context of the WCAG guidelines. 

Defining “Perceivable” Content in Web Accessibility

No matter how you present it — text, video, audio, graphics — your website is full of information. The guidelines in the first section of WCAG 2.1 help webmasters make that information available to everyone, regardless of how users consume content. And the perceivability principle doesn’t just apply to information; user interfaces must meet the same standards. 

To be considered perceivable, web content must reach users through the senses they rely on to receive information. That typically means multimodal presentation, so site visitors who listen to content don’t find themselves impeded by text alone, and visual readers aren’t stuck with an audio-only format, for example. 

In short, “perceivable” web content includes means of presentation that make information and functionality available to all users. The WCAG Success Criteria numbered 1.1 through 1.4.13 provide details on exactly what that means for website operators. 

Examples of WCAG Success Criteria for Perceivability

These Success Criteria provide practical advice for creating more accessible websites. While the following points aren’t comprehensive, they offer helpful context on the meaning of “perceivability” in WCAG. (For more information on WCAG Success Criteria for Perceivability, see the full text of WCAG 2.1 — and click here to learn about WCAG 2.2, expected to go into effect in 2022).

• If content isn’t text, include a text alternative that conveys the same information. That could include alt text tags for images; captions for audio and video content; or, at the very least, written explanations of elements that can’t be communicated in text (like CAPTCHAs, tests, and exercises). 
• Text content must contrast significantly with background color, and must maintain coherence when resized. Success Criteria 1.4.3 places the minimum brightness contrast between a background and text at 4.5:1 in most cases. Criteria 1.4.4 requires text to be resizable up to 200 percent without obscuring content or losing functionality. 
• Web pages must function in a variety of layouts and configurations without causing confusion. In addition to being mobile-friendly, websites must maintain a coherent structure in landscape or portrait view; in simplified views, such as text-only; and when font is enlarged or reduced. Input fields must keep labels intact, regardless of the display mode, and these changes shouldn’t jumble the reading sequence.  
• Be careful with audio autoplay. If your website automatically plays sound for more than three seconds, Success Criterion 1.4.2 requires a clear means to pause or stop the audio — or at least a way to control the volume outside of the device audio controls.  
• Outside of a few rare scenarios, use on-page text rather than images of text. Screen readers and other assistive technology might not recognize images of text. That’s why WCAG recommends text instead of images of text, unless the information can’t be conveyed without an image of text—or the site allows users to customize the image. 

To reiterate, this is just a sample of the WCAG Success Criteria dedicated to improving perceivability for accessible online content. Hopefully it’s enough to illustrate the principle at play in this section of the guidelines: Users perceive information differently, through many senses, and with unique requirements for maintaining engagement. To build an accessible website, content must be perceivable to all.