Check colour contrast

What "large text" means in WCAG

The W3C defines large text precisely to avoid ambiguity: 18.66px or larger at normal weight (400), OR 14px or larger when bold (700+). This distinction reflects the perceptual fact that bold text is harder to read than regular weight — the threshold can relax accordingly.

In practice, if your text is bigger than 18px, it counts as large. At 14–17px, it only qualifies if it’s bold. Below 14px, it must meet the normal-text threshold even if bold.

Contrast ratios are a proxy, not a guarantee

WCAG contrast math models relative luminance — how bright a colour appears — but it does not model colour perception holistically. Someone with deuteranopia (red-green colour blindness) may struggle with red-on-green pairs that pass WCAG AA mathematically, because the colours are perceptually similar to their eyes.

The formula catches most problematic pairs, but the best practice is to avoid relying on colour alone to convey information, and to test with actual users if accessibility is critical to your service.

APCA: the next standard

The Accessible Perceptual Contrast Algorithm (APCA) is a newer model adopted by the W3C's Accessibility Guidelines Working Group as the basis for WCAG 3. It aims to predict perceived contrast more accurately by accounting for spatial frequency, colour, and font size together in one formula.

APCA uses "Lc" units instead of simple ratios, and thresholds vary by text size: stricter (higher Lc required) for small text, relaxed for large text. It skips the somewhat arbitrary two-tier (AA/AAA) system in favour of one scale that adjusts to real-world readability.

Until WCAG 3 reaches Candidate Recommendation status, WCAG 2.1 contrast remains the legal accessibility standard. But APCA is worth studying if you want to ship interfaces that are genuinely easy to read.