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