WebAIM: Introduction to Web Accessibility - Web Accessibility In Mind introduces some of the principles of accessible design.
Web Content Accessibility Guidelines (WCAG) 2.1 - Web Content Accessibility Guidelines (WCAG) 2.1 covers a wide range of recommendations for making Web content more accessible.
WebAIM's WCAG 2 Checklist - a checklist of WCAG guidelines and success criteria.
WAI-ARIA Overview | Web Accessibility Initiative (WAI) | W3C - WAI-ARIA defines a way to make Web content and Web applications more accessible to people with disabilities. It helps with dynamic content, and advanced user interface controls developed with HTML, JavaScript, and related technologies.
ARIA Authoring Practices Guide | APG | WAI | W3C - This guide describes how to apply accessibility semantics to common design patterns and widgets.