APCA (Advanced Perceptual Contrast Algorithm)
Advanced Perceptual Contrast Algorithm, perceptual contrast algorithm proposed for WCAG 3, based on human perception of luminance and polarity (light text on dark vs reverse) rather than the simple mathematical ratio in WCAG 2 (4.5:1 / 3:1). More representative of actual readability, but removed from the WCAG 3 working draft in July 2023, then put back in exploratory mode. Per Adrian Roselli (April 2026 article), no WCAG 3 contrast algorithm will be final before 2030 at the earliest.
Strengths
- More representative of human perception than the WCAG 2 mathematical ratio
- Accounts for polarity (light text on dark vs reverse)
- Implemented in Stark, ColorContrast.app, HSLuv, Accessible Palette, growing adoption
Limitations
- No final standard before 2030 at the earliest (Roselli 2026), risk of investing in a moving target algorithm
- Removed from WCAG 3 working draft in July 2023, not a W3C standard today
- More complex to explain and apply than a simple ratio, pedagogical barrier
Best for
- DS managers wanting to understand where WCAG 3 is going and prepare their color tokens
- Designers and a11y experts pushing beyond the simple WCAG 2 ratio