The four principles (POUR)
Perceivable: information must be presented so people can sense it. Provide text alternatives for images and captions for audio and video.
Operable: every control must work with a keyboard, users must have enough time to act, and navigation must be predictable.
Understandable: text must be readable, and pages must behave in ways users expect, with helpful error messages.
Robust: content must work with a wide range of browsers and assistive technologies, both now and in the future.
Practical checks
Give every meaningful image an accurate alt description, and give decorative images an empty alt attribute so screen readers skip them.
Make sure text has enough contrast against its background, and never rely on colour alone to convey meaning.
Confirm that you can reach and operate every button, link, and form field using only the Tab, Enter, and arrow keys, and that the focus indicator is always visible.