Colorable - An easy to understand colour contrast checker by Brent Jackson

Usecontrast - a macOS app for quick axxess to WCAG colour contrast ratios

A Good Basis for Accessibility - This article on MDN dives deep into semantic HTML as a basis for building accessible sites

HTML accessibility - maintained by @stevefaulkner gives an overview of browser support for HTML5 features across browsers.

Accessibility through Semantic HTML - Laura Kalbag dives into how we can build accessible sites by simply sticking to semantic HTML. Really worth the read.

Web Accessibility by Google - A free web accessibility course by Google on Udacity.

About HTML semantics and front-end architecture - @necolas (the author of normalise.css) super timely article, a great read.

HTML5 Hidden Attribute - A short article on using the hidden attribute rather than using CSS

Where to put Buttons in Forms - Great do's and don'ts for forms