Perceivable -- WCAG Principle 1

Information and user interface components must be presentable to users in ways they can perceive.

What Does "Perceivable" Mean?

Perceivable is the first of the four top-level principles in WCAG 2.2 [1], which organise its success criteria under the POUR acronym -- Perceivable, Operable, Understandable, Robust. The principle states that information and user interface components must be presentable to users in ways they can perceive. If a user has no sensory channel to receive a piece of content, the content does not exist for them regardless of how well the rest of the page is built.

The failure mode is concrete: a control whose state is conveyed only through hue leaves a dichromatic user without the signal; an icon button whose only accessible name is a decorative glyph exposes nothing meaningful to a screen reader; a video shipped without a caption track withholds all spoken content from a deaf or hard-of-hearing viewer; text rasterised into a background image cannot be resized, recoloured, or reflowed by the user agent. Each is a perceivability failure because the information is present in the page but gated behind a single sensory channel that some users cannot use.

The principle decomposes into four guidelines. 1.1 Text Alternatives requires that non-text content carry a programmatic text equivalent so assistive technologies can translate it into speech, braille, symbols, or enlarged print. 1.2 Time-based Media covers prerecorded and live audio and video, specifying captions, audio description, transcripts, and sign-language tracks at A, AA, and AAA conformance levels. 1.3 Adaptable requires that structure and relationships conveyed visually also be exposed in the markup, so users can re-present the content without losing meaning. 1.4 Distinguishable governs perceptual qualities -- contrast, text sizing, reflow, spacing, audio control, and non-text contrast -- that determine whether foreground content separates from background at the user's settings. Each guideline expands into individual success criteria rated A (must), AA (should for most legal targets), or AAA (aspirational).

Guidelines Under Perceivable

1.1 Text Alternatives

Guideline 1.1 contains a single success criterion, 1.1.1 Non-text Content (Level A) [2]. The requirement is that every non-text element -- images, icons, charts, audio, CAPTCHAs, decorative media -- carries a programmatic text equivalent suited to its purpose, or is marked to be ignored by assistive technology when it is purely decorative. The text alternative is the pivot that lets the same content be rendered as speech, braille, symbols, or large print.

1.2 Time-based Media

Guideline 1.2 spans nine success criteria across A, AA, and AAA. For prerecorded media, Level A requires a text or media alternative for audio-only and video-only content [3] and captions for any synchronised audio [4]; Level AA adds audio description of visual information [5]. For live content, Level AA requires live captions [6]. The AAA tier layers on sign-language interpretation, extended audio description, and full media alternatives. Captions carry dialogue plus non-speech audio cues; audio description narrates visual information that is not spoken in the soundtrack -- the two cover different sensory gaps and are not interchangeable.

1.3 Adaptable

Guideline 1.3 is the semantic-structure guideline. 1.3.1 Info and Relationships (Level A) [7] requires that structure conveyed visually -- headings, lists, tables, form labels, grouped controls -- also be exposed in the accessibility tree via native HTML or ARIA, so a user who disables CSS, enlarges the viewport, or listens rather than looks still receives the relationships. 1.3.2 Meaningful Sequence [8] requires that DOM order match the reading order. 1.3.3 Sensory Characteristics [9] bans instructions that rely solely on shape, size, colour, or position ("click the round button on the right"). 1.3.4 Orientation [10] and 1.3.5 Identify Input Purpose [11] round out Level AA. Adaptability is what lets the same markup survive user stylesheets, reflow, zoom, and screen-reader linearisation without losing meaning.

1.4 Distinguishable

Guideline 1.4 is the largest under Perceivable and governs the perceptual qualities that determine whether content separates from its background at the user's settings. 1.4.1 Use of Color [12] forbids colour as the sole carrier of meaning. 1.4.3 Contrast (Minimum) [13] sets the 4.5:1 / 3:1 text contrast floor at Level AA, and 1.4.11 Non-text Contrast [14] extends a 3:1 floor to UI component boundaries and graphical objects. 1.4.4 Resize Text [15] and 1.4.10 Reflow [16] require that content scale to 200% and reflow to a 320 CSS pixel viewport without two-dimensional scrolling. 1.4.12 Text Spacing [17] requires that user-applied line, word, letter, and paragraph spacing not break layout. 1.4.13 Content on Hover or Focus [18] constrains tooltips and popovers so they can be dismissed, hovered, and persisted. 1.4.2 Audio Control [19] handles auto-playing audio. Together these criteria mean that a user running a high-contrast stylesheet, a 400% zoom, a reader mode with forced spacing, or a keyboard-only interaction model still gets a page whose foreground is perceptually distinct from its background.

References

  1. [1] W3C (2023). Web Content Accessibility Guidelines (WCAG) 2.2. W3C, Accessed 2026-04-07. https://www.w3.org/TR/WCAG22/ ↩
  2. [2] W3C (2023). Understanding Success Criterion 1.1.1: Non-text Content. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/non-text-content.html ↩
  3. [3] W3C (2023). Understanding Success Criterion 1.2.1: Audio-only and Video-only (Prerecorded). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/audio-only-and-video-only-prerecorded.html ↩
  4. [4] W3C (2023). Understanding Success Criterion 1.2.2: Captions (Prerecorded). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/captions-prerecorded.html ↩
  5. [5] W3C (2023). Understanding Success Criterion 1.2.5: Audio Description (Prerecorded). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/audio-description-prerecorded.html ↩
  6. [6] W3C (2023). Understanding Success Criterion 1.2.4: Captions (Live). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/captions-live.html ↩
  7. [7] W3C (2023). Understanding Success Criterion 1.3.1: Info and Relationships. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/info-and-relationships.html ↩
  8. [8] W3C (2023). Understanding Success Criterion 1.3.2: Meaningful Sequence. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/meaningful-sequence.html ↩
  9. [9] W3C (2023). Understanding Success Criterion 1.3.3: Sensory Characteristics. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/sensory-characteristics.html ↩
  10. [10] W3C (2023). Understanding Success Criterion 1.3.4: Orientation. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/orientation.html ↩
  11. [11] W3C (2023). Understanding Success Criterion 1.3.5: Identify Input Purpose. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/identify-input-purpose.html ↩
  12. [12] W3C (2023). Understanding Success Criterion 1.4.1: Use of Color. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/use-of-color.html ↩
  13. [13] W3C (2023). Understanding Success Criterion 1.4.3: Contrast (Minimum). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html ↩
  14. [14] W3C (2023). Understanding Success Criterion 1.4.11: Non-text Contrast. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/non-text-contrast.html ↩
  15. [15] W3C (2023). Understanding Success Criterion 1.4.4: Resize Text. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/resize-text.html ↩
  16. [16] W3C (2023). Understanding Success Criterion 1.4.10: Reflow. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/reflow.html ↩
  17. [17] W3C (2023). Understanding Success Criterion 1.4.12: Text Spacing. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/text-spacing.html ↩
  18. [18] W3C (2023). Understanding Success Criterion 1.4.13: Content on Hover or Focus. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/content-on-hover-or-focus.html ↩
  19. [19] W3C (2023). Understanding Success Criterion 1.4.2: Audio Control. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/audio-control.html ↩