Accessibility is a design constraint, not a checklist
Treated as an audit at the end, it produces compliance and little else. Treated as a constraint, it usually produces a better interface.
Accessibility work that begins after the design is signed off tends to produce a list of remediations: contrast bumped a little, labels added, focus outlines restored after someone removed them.
The result passes, and the experience is still worse than it should be for everyone.
Constraints improve design
Requiring sufficient contrast pushes you towards a palette that survives a bright afternoon on a phone. Requiring keyboard operability forces a sane focus order, which is also a sane reading order.
The things most often missed
Focus states removed for looking untidy, breaking keyboard use entirely
Colour used as the only signal for an error or a status
Headings chosen for size rather than structure, leaving no usable outline
Click handlers on non-interactive elements, invisible to assistive technology
Text over imagery where contrast depends on which photograph loaded
Test cheaply and often
Put the mouse away and try to complete your primary journey with the keyboard alone. Most problems surface within a minute, long before anyone runs a formal audit.