Helpful links:
WebAIM – Web Accessibility in Mind: https://webaim.org/
WebAIM Contrast Checker Tool https://webaim.org/resources/contrastchecker/
aXe – automated accessibility testing: https://axe-core.org/
a11y Styleguide / Pattern Library: http://a11y-style-guide.com/style-guide/
Paciello Group: https://www.paciellogroup.com/
W3C WAI ARIA Pratices: https://www.w3.org/TR/wai-aria-practices/