Accessibility is not a checklist bolted on at the end; it is a lens on every other topic here. Sufficient color contrast, legible type, visible focus, respectful motion, and semantic structure are what make a design usable by everyone. Treat it as craft, not compliance.
A clear, authoritative introduction to web accessibility and the POUR principles.
Accessibility guidance is often either legalistic or hand-wavy; WebAIM is neither, giving you the principles and the concrete practices in plain language. Start here to understand contrast, semantic structure, keyboard focus, and why they matter, then use their contrast checker in daily work. It is the most trustworthy free on-ramp to the topic.