Color Contrast Accessibility Skill

Canonical source: examples/COLOR_CONTRAST_ACCESSIBILITY_BEST_PRACTICES.md in mgifford/ACCESSIBILITY.md This skill is derived from that file. When in doubt, the example is authoritative.

Apply these rules whenever implementing or reviewing colour choices in HTML, CSS, SVG, or any visual interface element.

Core Mandate

Sufficient contrast between foreground and background colors is a prerequisite for users to read text, identify UI components, perceive graphical content, and track keyboard focus. Color alone must never be the sole means of conveying information.

All visual interface elements that convey information or require user interaction must meet WCAG 2.2 Level AA contrast thresholds in light mode, dark mode, and forced-colors (high contrast) mode.

Severity Scale (this skill)

Critical: Text Contrast (WCAG 1.4.3)

Normal text and images of text must meet these minimums:

"Large text" means 18pt (≈ 24 CSS px) or larger in regular weight, or 14pt (≈ 18.67 CSS px) or larger in bold weight.

Preferred CSS pattern — text colours via custom properties

:root {
  --color-text:        #1a1a1a;   /* contrast vs #fff: 16.75:1 ✓ */
  --color-text-muted:  #595959;   /* contrast vs #fff:  7.0:1  ✓ */
  --color-heading:     #333333;   /* contrast vs #fff: 12.63:1 ✓ */
  --color-link:        #0066cc;   /* contrast vs #fff:  4.52:1 ✓ */
  --color-background:  #ffffff;
}

@media (prefers-color-scheme: dark) {
  :root {
    --color-text:        #e8e8e8;   /* contrast vs #1a1a1a: 13.61:1 ✓ */
    --color-text-muted:  #b0b0b0;   /* contrast vs #1a1a1a:  7.0:1  ✓ */
    --color-link:        #66aaff;   /* contrast vs #1a1a1a:  5.74:1 ✓ */
    --color-background:  #1a1a1a;
  }
}

Avoid

/* Bad — 2.4:1 contrast, fails 1.4.3 */
.placeholder { color: #aaaaaa; }

/* Bad — informative text in low-contrast colour */
.note { color: #888; } /* fails against white background */

Serious: Non-text Contrast (WCAG 1.4.11)

UI components and graphical objects required to understand or operate the interface must have 3:1 contrast against adjacent colours.

Applies to

• Form input borders (text fields, checkboxes, radio buttons, selects)
• Interactive component boundaries (buttons without text, sliders, toggles)
• Icons and graphical objects that convey meaning
• Charts and data visualisation elements that encode information
• Status indicators (progress bars, meter fills)

Does not apply to

• Decorative graphics that convey no meaning
• Inactive / disabled components
• Logos and brand marks
• Graphical elements supplementary to adjacent text

Form control pattern

/* Checkbox border must contrast 3:1 against its background */
input[type="checkbox"] {
  --checkbox-border: #767676; /* 4.54:1 against #fff ✓ */
  appearance: none;
  width: 1.25rem;
  height: 1.25rem;
  border: 2px solid var(--checkbox-border);
  border-radius: 3px;
}

Serious: Use of Color (WCAG 1.4.1)

Color alone must not be the sole means of conveying information, indicating an action, prompting a response, or distinguishing a visual element. A second, non-color cue must always accompany color.

Common failure patterns

<!-- Bad: required field indicated only by red label color -->
<label style="color: red;">Email address</label>
<input type="email">

<!-- Bad: error state communicated only by red border -->
<input type="email" style="border-color: red;">

Preferred patterns

<!-- Good: required field — asterisk + color + screen-reader text -->
<label>
  Email address
  <span aria-hidden="true" class="required-marker">*</span>
  <span class="sr-only">(required)</span>
</label>
<input type="email" aria-required="true">

<!-- Good: error state — icon + text + color + aria-invalid -->
<div class="field field--error">
  <label for="email">Email address</label>
  <input id="email" type="email"
         aria-describedby="email-error" aria-invalid="true">
  <p id="email-error" class="error-message">
    <svg role="img" aria-label="Error" aria-hidden="true">
      <use href="#icon-exclamation"></use>
    </svg>
    Please enter a valid email address.
  </p>
</div>

Link distinction from surrounding text

Links within body text must be distinguishable from surrounding text by more than color alone. Use underline (the browser default) or another non-color cue.

/* Good: underline preserved */
a {
  color: #0066cc;
  text-decoration: underline;
}

Serious: Focus Appearance (WCAG 2.4.13)

WCAG 2.2 2.4.13 Focus Appearance (Level AA) requires visible keyboard focus indicators that:

1. Enclose the focused component with an area of at least the perimeter × 2 CSS px in thickness.
2. Have 3:1 contrast between focused and unfocused states.
3. Have 3:1 contrast against every adjacent color in the unfocused state.

Preferred CSS pattern

:root {
  --focus-ring-color:  #0066cc;
  --focus-ring-width:  3px;
  --focus-ring-offset: 2px;
}

:focus-visible {
  outline: var(--focus-ring-width) solid var(--focus-ring-color);
  outline-offset: var(--focus-ring-offset);
  /* White halo ensures visibility on dark backgrounds */
  box-shadow: 0 0 0 calc(var(--focus-ring-width) + var(--focus-ring-offset))
              #ffffff;
}

@media (prefers-color-scheme: dark) {
  :root {
    --focus-ring-color: #99ccff;
  }
}

C40 two-color focus indicator

/* Works on both light and dark surfaces */
:focus-visible {
  outline: 3px solid #000000;
  outline-offset: 1px;
  box-shadow: 0 0 0 5px #ffffff;
}

Avoid

/* NEVER do this — completely removes focus visibility */
:focus { outline: none; }

/* NEVER do this without providing an alternative focus style */
*:focus { outline: 0 !important; }

Serious: Forced-Colors Mode (WCAG 1.4.3, 1.4.11)

Windows High Contrast Mode and forced-colors replace author colors with system colors. Interfaces break when CSS background-color, box-shadow, or color properties are the sole means of conveying meaning.

Use outline for focus rings — it survives forced-colors

/* Good: outline is forced-colors-safe */
:focus-visible {
  outline: 3px solid Highlight;
  outline-offset: 2px;
}

/* Risk: box-shadow may not render in forced-colors mode */
:focus-visible {
  box-shadow: 0 0 0 3px #0066cc; /* may be suppressed */
}

Restore lost meaning in forced-colors mode

@media (forced-colors: active) {
  .button {
    background-color: ButtonFace;
    color: ButtonText;
    border: 2px solid ButtonBorder;
  }

  /* Restore SVG icon visibility */
  .icon {
    forced-color-adjust: auto;
  }
}

Testing forced-colors mode

• Enable High Contrast Mode in Windows Accessibility settings
• Chrome DevTools → Rendering → "Emulate CSS media feature forced-colors: active"
• Firefox: about:config → ui.forcedColors: 1

Moderate: Semantic Color Token Pattern

Centralizing all design-system colors as CSS custom properties makes contrast validation and theming manageable at scale.

/* Token layer — raw values */
:root {
  --color-neutral-600: #595959;  /* 7.0:1 on #fff */
  --color-neutral-900: #1a1a1a;  /* 16.75:1 on #fff */
  --color-brand-500:   #0066cc;  /* 4.52:1 on #fff — OK for normal text ✓ */
  --color-brand-700:   #004c99;  /* 7.59:1 on #fff — OK for all text  ✓ */

  /* Semantic layer */
  --color-text-primary:   var(--color-neutral-900);
  --color-text-secondary: var(--color-neutral-600);
  --color-text-link:      var(--color-brand-500);
  --color-surface:        #ffffff;
  --color-border:         #e8e8e8; /* 3:1 for non-text ✓ */
}

@media (prefers-color-scheme: dark) {
  :root {
    --color-text-primary:   #e8e8e8;
    --color-text-secondary: #a0a0a0;  /* 7.11:1 on #1a1a1a ✓ */
    --color-text-link:      #66aaff;  /* 5.74:1 on #1a1a1a ✓ */
    --color-surface:        #1a1a1a;
    --color-border:         #444444;  /* 3.1:1 vs #1a1a1a ✓ */
  }
}

/* Good: references semantic token — works in all themes */
.card {
  background-color: var(--color-surface);
  color: var(--color-text-primary);
  border: 1px solid var(--color-border);
}

/* Bad: hard-coded value that may fail in dark mode */
.card {
  background-color: #ffffff;
  color: #333;
}

Minor: APCA — Emerging Standard

The Advanced Perceptual Contrast Algorithm (APCA) is a candidate replacement for the WCAG 2.x contrast ratio formula expected in WCAG 3.0. It models contrast perception more accurately for thin strokes, small font sizes, and saturated colors.

APCA is not yet required. Teams adopting it today must continue to meet WCAG 2.2 AA requirements in parallel.

Recommended Contrast-Checking Tools

Testing programmatically with axe-core

const axe = require("axe-core");

axe.run(document, {
  runOnly: {
    type: "rule",
    values: ["color-contrast", "color-contrast-enhanced"]
  }
}, (err, results) => {
  if (err) throw err;
  console.log("Contrast violations:", results.violations);
});

Definition of Done Checklist

• [ ] All normal text meets 4.5:1 contrast in light and dark modes
• [ ] All large text meets 3:1 contrast in light and dark modes
• [ ] All form controls, icons, and data visualisation elements meet 3:1 non-text contrast
• [ ] No information conveyed by color alone — icon + text + color used together
• [ ] Links in body text are distinguishable from surrounding text without color
• [ ] Focus indicators use outline, meet 3:1 contrast, and are visible in all modes
• [ ] All colour tokens defined as CSS custom properties; no hard-coded values in components
• [ ] Light mode and dark mode verified with contrast checker
• [ ] Forced-colors / Windows High Contrast Mode tested — no meaning lost
• [ ] axe-core color-contrast rule passes in CI
• [ ] Disabled controls clearly distinguished from enabled controls beyond opacity alone

Key WCAG Criteria

• 1.4.1 Use of Color (A) — Serious if failing
• 1.4.3 Contrast Minimum (AA) — Critical if failing for normal text
• 1.4.6 Contrast Enhanced (AAA)
• 1.4.11 Non-text Contrast (AA) — Serious if failing
• 2.4.7 Focus Visible (AA)
• 2.4.13 Focus Appearance (AA, WCAG 2.2) — Serious if failing

References

• Full best practices guide
• WCAG 2.2 Understanding 1.4.1 Use of Color
• WCAG 2.2 Understanding 1.4.3 Contrast (Minimum)
• WCAG 2.2 Understanding 1.4.11 Non-text Contrast
• WCAG 2.2 Understanding 2.4.13 Focus Appearance
• WCAG 2.2 Technique C40: Creating a two-color focus indicator
• APCA contrast tool

Standards horizon: WCAG 3.0's proposed APCA (Advanced Perceptual Contrast Algorithm) will replace the current luminance-ratio model with a perceptual contrast model that treats light-on-dark and dark-on-light differently. Do not apply APCA to production work until WCAG 3.0 is a published standard. Monitor: <https://www.w3.org/TR/wcag-3.0/> and <https://git.apcacontrast.com/>