Color contrast should use the APCA algorithm for determining contrast (or both algorithms)
Introduction
The Accessible Perceptual Contrast Algorithm (APCA) is a modern, context-dependent method for calculating color contrast designed for WCAG 3.0.
Decision
Promote the use of APCA algorithm for determining color contrast.
Context
Unlike older standards (WCAG 2.x) that use a fixed ratio, APCA evaluates readability based on human perception, text size, font weight, background luminance, and polarity (light text on dark background vs. dark on light).
Smaller fonts or text with a lighter weight require more contrast to be legible, which APCA builds directly into the mathematical equation rather than treating it as a separate checklist item.
Traditional WCAG 2.x ratios often pass combinations for bright text on dark backgrounds that actually cause eye strain and visual artifacts (halation). APCA corrects for this.
Consequences
- Better readability and less eye strain, which overall is better for accessibility.
- Some accessibility scanners may flag color combinations that are considered accessible using the new algorithm, so it's probably better to use colors that pass both systems.
- Designers may not be using this standard, so developers may need to check and provide feedback.
- New algorithm may not pass existing brand colors.
Exceptions
- Designers or clients insist on using colors that only pass existing WCAG standards with the old algorithm.