Contrast (Minimum)
Body text must have a luminance contrast ratio of at least 4.5:1 against its background; large text (≥18pt or ≥14pt bold) needs at least 3:1.
What it requires
Color contrast is the most-violated WCAG criterion on the web — affecting 81% of homepage scans according to WebAIM 2024. Pale-grey body text on white backgrounds, brand colors that look fine to designers but fail the 4.5:1 luminance threshold, and disabled-state buttons that are too dim are the most common failures.
The luminance formula is defined in WCAG and not equivalent to color difference or hue rotation. Tools like WebAIM Contrast Checker and Colour Contrast Analyser compute it correctly.
Common Shopify failure
Default theme colors set to `#999` body text on `#fff`, which yields a 2.85:1 ratio (failing). Brand orange buttons on white that fall to 3.4:1 (passing for large but failing if used on small captions).
How to fix it
AccessComply's ColorContrastAgent finds the closest brand-aligned color that meets the threshold — typically darkening lightness by 5-15% while preserving hue — and writes the change as a CSS custom property override at the top of the theme's base.css.
Primary source: W3C — WCAG 2.0 Understanding 1.4.3