The WebAIM Million: An Accessibility Analysis of the Top 1,000,000 Home Pages
https://webaim.org/projects/million/
Method
"The WAVE accessibility engine was used to analyze the rendered DOM (Document Object Model) of all pages after scripting and styles were applied."
Population
"the Tranco ranking which combines several sources of 'top' web sites."
What it does not show
The engine flags mechanically detectable markup failures only. It does not test usability with assistive technology, so the count undercounts real barriers and a clean scan does not mean a page is usable. Home pages only — nothing about interior pages, authenticated flows or mobile apps.
WebAIM, Utah State University
95.9% of one million home pages had automatically detectable WCAG 2 conformance failures, averaging 56.1 errors per page. Six categories account for 96% of all detected errors and have done for seven consecutive years: low-contrast text (83.9% of pages), missing alternative text (53.1%), missing form labels (51%), empty links (46.3%), empty buttons (30.6%), missing document language (13.5%).
Tier II: Large-N automated census of a million real production home pages. Observational and descriptive with no comparison group.