The Four Principles (POUR)
The Web Content Accessibility Guidelines (WCAG) 2.1are organized around four principles. Every requirement falls under one of these ideas — remembered by the acronym POUR:
WCAG defines three levels of conformance: Level A(minimum), Level AA (the standard required by the ADA web rule), and Level AAA (highest). The ADA rule requires both Level A and Level AA success criteria.
Perceivable
People must be able to see, hear, or otherwise sense all the content on a page. Key requirements:
- Text alternatives for images: Every image needs alt text that describes its purpose. A photo of a building entrance might have: "Accessible entrance with automatic doors at City Hall."
- Captions for video: All prerecorded videos need accurate captions. Live streams also need real-time captions at Level AA.
- Audio descriptions: Videos that convey important visual information (charts, actions, on-screen text) need audio descriptions for blind users.
- Color contrast: Text must have a 4.5:1 contrast ratio against its background. Large text (18pt or 14pt bold) needs only 3:1. Light gray text on white backgrounds is a common failure.
- Don't rely on color alone: If a required field is shown only by turning red, a screen reader user won't know it's required. Use labels, icons, or text in addition to color.
Official Standard
WCAG 2.1 Principle 1 — Perceivable
1.1 Text Alternatives: "All non-text content that is presented to the user has a text alternative that serves the equivalent purpose."
1.2 Time-Based Media: Provide alternatives for time-based media, including captions for prerecorded and live audio, and audio descriptions for prerecorded video.
1.3 Adaptable: "Create content that can be presented in different ways without losing information or structure."
1.4.3 Contrast (Minimum — AA): "The visual presentation of text and images of text has a contrast ratio of at least 4.5:1," except for large text (3:1), incidental text, and logotypes.
Operable
People must be able to use the website — navigate it, fill out forms, click buttons, and interact with all features.
- Keyboard accessible: Every interactive element — links, buttons, forms, menus, sliders — must work with a keyboard alone. Many people can't use a mouse due to mobility impairments.
- No keyboard traps: A user navigating with Tab must never get stuck in a component they can't leave.
- Visible focus indicator: When someone tabs through a page, there must be a visible outline or highlightshowing which element has focus.
- Enough time: If content has a time limit (like a session timeout), users must be able to extend it.
- No seizure-inducing content: Nothing on the page should flash more than 3 times per second.
- Touch targets (new in 2.1): Complex gestures (like pinch-to-zoom) must have single-pointer alternatives.
Official Standard
WCAG 2.1 Principle 2 — Operable
2.1 Keyboard Accessible: "All functionality of the content is operable through a keyboard interface without requiring specific timings for individual keystrokes."
2.4.7 Focus Visible (AA): "Any keyboard operable user interface has a mode of operation where the keyboard focus indicator is visible."
2.5.1 Pointer Gestures (AA — WCAG 2.1): "All functionality that uses multipoint or path-based gestures for operation can be operated with a single pointer without a path-based gesture."
2.3.1 Three Flashes or Below Threshold:"Web pages do not contain anything that flashes more than three times in any one second period."
Understandable
Content and navigation must be clear and predictable. Users shouldn't be surprised or confused by how the site works.
- Page language declared: The HTML must specify the language (e.g.,
lang="en") so screen readers pronounce words correctly. - Predictable behavior: The page should not unexpectedly change when a user focuses on something or fills in a form field. For example, selecting a dropdown option should not automatically submit a form.
- Clear error messages: When a form has errors, the site must clearly identify which field has the problem and describe the error in text. At Level AA, it must also suggest how to fix it.
- Consistent navigation: Navigation menus should appear in the same location on every page.
Official Standard
WCAG 2.1 Principle 3 — Understandable
3.1.1 Language of Page (A): "The default human language of each Web page can be programmatically determined."
3.2.1 On Focus (A): "When any user interface component receives focus, it does not initiate a change of context."
3.3.1 Error Identification (A): "If an input error is automatically detected, the item that is in error is identified and the error is described to the user in text."
3.3.3 Error Suggestion (AA): "If an input error is automatically detected and suggestions for correction are known, then the suggestions are provided to the user."
Robust
Content must work reliably with current and future technologies, including assistive technologies like screen readers, magnifiers, and voice control software.
- Valid HTML: Code must be well-formed — proper opening and closing tags, no duplicate IDs, and correct nesting. Broken HTML confuses assistive technology.
- Name, role, value: Every user interface component (button, form field, tab, slider) must expose its name (what it is), role (what it does), and value/state (its current setting) to assistive technology.
- Status messages: When something changes on the page (like a success confirmation or an error count), screen reader users must be notified without losing their place.
WCAG 2.1, Guideline 4.1
"Maximize compatibility with current and future user agents, including assistive technologies." This principle ensures content remains accessible as technology evolves.
Level A vs. AA vs. AAA
WCAG organizes its requirements into three levels:
Common Failures with Examples
These are the accessibility issues found most oftenon government and business websites:
Important Note
This guide provides general information about WCAG 2.1 Level AA requirements. Web accessibility is a technical and legal area that benefits from professional evaluation. For legal advice about your obligations or your rights, connect with an experienced ADA attorney.
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