Glossary · Accessibility term
Perceivable
Perceivable is the first of WCAG's four principles, and the standard states it as information and user interface components being presentable in ways people can perceive. The second half of that sentence is the half that gets dropped, and it is why the contrast rule for buttons and icons lives here alongside the one for text. In practice it means images carry text alternatives, and audio and video carry captions and descriptions. Structure sits in the markup rather than only in the styling. And nothing depends on colour alone or falls apart when somebody enlarges the text. It holds 29 criteria across four guidelines, spread unevenly in a way worth knowing. Text alternatives is a single criterion carrying that whole apparatus, while the guideline about telling content apart from its background holds 13, seven of them at Level AA. That is where most of the AA work on a normal site turns out to be.
In practice
Perceivable is where the volume of detected failure sits, which is not quite the same as saying most failures are here. Six error types account for 96% of everything automated tools find, and the two at the top of that list belong to this principle. Low contrast text on 83.9% of home pages, and missing alt text on 53.1%. Contrast leads by a distance. Missing alt text is only two points clear of the missing form labels behind it, so treat the second place as a photo finish rather than a gap.
The alt text picture is worse than the headline number suggests. More than one in four images on popular home pages has alternative text that is missing, questionable or repetitive. Questionable takes in the word image, the word graphic, a file name, and text identical to whatever sits next to it. Nearly half the images missing alt text were inside links, which leaves the link leaning on whatever other text it has, and often it has none.
One guideline is missing from most people's mental picture of this principle and it carries the criterion audits cite most. Adaptable is about structure surviving presentation, which covers headings being real headings, labels being connected to their fields, and the sequence in the code still carrying whatever the sequence on screen was carrying. The two orders do not have to be identical. What has to survive is the meaning. None of that is about the senses in the obvious way, and all of it is here.
Why it matters
This is the principle where a scanner does the most useful work, and the same data shows how far that gets you. Detected failures were found on 95.9% of the top million home pages in February 2026, up from 94.8% a year before, which reversed six years of small improvements. WebAIM's own reading is that pages are getting larger and more complex faster than anybody is fixing them, with average elements per home page up 22.5% in a single year. Four of the six commonest failures got worse rather than better that year, and contrast led the reversal, from 79.1% of home pages to 83.9%. It is also the one on the list a machine detects most reliably and a design decision creates. Fix the contrast first.
What it covers
Anything that has to reach the senses at all. Alt text, captions, audio description, contrast, resize and reflow all sit here. So do headings and labels, because a structure that exists only in the styling has not reached anybody who is not looking at it.
Where this shows up on the site
Related terms
- POURPOUR is the field's shorthand for WCAG's four principles, which are Perceivable, Operable, Understandable and Robust.
- OperableOperable is the second WCAG principle, and its one-line statement is that user interface components and navigation must be operable.
- UnderstandableUnderstandable is the third of WCAG's four principles, and it splits in two the way its own sentence does.
- RobustRobust is the fourth of WCAG's four principles, and it asks that content be built well enough for a wide variety of user agents, including assistive technology, to interpret it.
- Alt textAlt text is the written description attached to an image, and it is what a screen reader reads out in place of the picture.
- Contrast ratioA contrast ratio is a number from 1:1 to 21:1 measuring how far apart two colors are in brightness, computed from a formula rather than judged by eye.
Knowing the word is the easy part.
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