WCAG 2.1 AA, explained in plain English
WCAG (Web Content Accessibility Guidelines) is the published standard for making websites usable by people with disabilities. Version 2.1 at level AA is the version most often referred to when a website is reviewed for accessibility.
Who writes it and what "AA" means
WCAG is published by the World Wide Web Consortium (W3C). Each requirement is called a success criterion, and each one has a level: A (the minimum), AA, or AAA (the strictest).
Meeting level AA means meeting every level A and every level AA success criterion. In WCAG 2.1 that is 50 criteria: 30 at level A and 20 at level AA.
The four principles
Every success criterion sits under one of four principles, often shortened to POUR:
- Perceivable: people can see or hear the content, for example images have text alternatives and text has enough contrast.
- Operable: people can use the site with a keyboard alone, and nothing flashes or times out without warning.
- Understandable: labels, instructions and error messages are clear, and pages behave predictably.
- Robust: the code is clean enough for screen readers and other assistive tools to interpret it.
What 2.1 added over 2.0
WCAG 2.1 kept everything in 2.0 and added criteria aimed mostly at phones and low-vision users: content must work in both portrait and landscape, text must reflow at 400% zoom without sideways scrolling, and non-text elements such as buttons and form borders need enough contrast.
What a scanner can check, and what it cannot
Many criteria are mechanical and a tool can test them reliably: color contrast, missing image text, form fields with no label, buttons with no name, a missing page language.
Others need a person. A tool can tell that an image has alt text, but not whether the text actually describes the image. It cannot judge whether a video caption is accurate or whether a page makes sense when read aloud. That is why an automated scan is a starting point, not a verdict.
Automated scans catch roughly a third to a half of WCAG criteria; the rest needs a person. SiteGuard is monitoring and a fix list, not a compliance certificate, legal advice or lawsuit protection.
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Last reviewed 2026-10-12.