ARIA Authoring Practices Guide (APG)
The ARIA Authoring Practices guide from the W3C Web Accessibility Initiative is an amazing resource for anyone tinkering on the web. It provides design patterns, examples, and practical guidance on making a webpage accessible according to the Accessible Rich Internet Application (ARIA) specification. The W3C Web Accessibility Initiative also has an introduction to web accessbility available here which provides a lot of great context for the items in their authoring practices guide.
Their patterns guide is an excellent quick reference to have nearby when working on any website . I use this page during my work life as a web developer quite often to ensure I am applying ARIA roles to elements properly and following best practices.