Colour Contrast on the Web: A WCAG 2.1 Level AA Compliance Audit of Common Crawl's Top 500 Domains

TL;DR

This study conducts a large-scale, automated audit of colour contrast compliance on the top 500 websites, revealing widespread accessibility issues and significant variation across categories.

Contribution

It introduces a reproducible, static CSS analysis method to assess WCAG 2.1 Level AA compliance without live crawling.

Findings

40.9% of colour pairs failed contrast standards

Median site pass rate was 62.7%

20.4% of sites achieved full compliance

Abstract

We present a large-scale automated audit of WCAG 2.1/2.2 Level AA colour contrast compliance across the 500 most frequently crawled registered domains in Common Crawl's CC-MAIN-2026-08 February 2026 crawl archive. Rather than conducting a live crawl, all page content was sourced from Common Crawl's open WARC archives, ensuring reproducibility and eliminating any load on target web servers. Our static CSS analysis of 240 homepages identified 4,327 unique foreground/background colour pairings, of which 1,771 (40.9%) failed to meet the 4.5:1 contrast ratio threshold for normal text. The median per-site pass rate was 62.7%, with 20.4% of sites achieving full compliance across all detected colour pairings. These findings suggest that colour contrast remains a widespread accessibility barrier on the most prominent websites, with significant variation across domain categories.