Accessible design means a product genuinely works for people with visual, motor, auditory, or cognitive differences — not an edge case, but a real and large share of any product's actual users. The WCAG (Web Content Accessibility Guidelines) is the standard reference; this lesson covers the essentials a designer touches daily.
WCAG defines minimum contrast ratios between text and its background — commonly 4.5:1 for normal text, 3:1 for large text, at the "AA" level most products target. Low-contrast light-grey-on-white text may look modern, but it's genuinely unreadable for many users, not a stylistic preference to defend.
Roughly 1 in 12 men have some form of color vision deficiency. A form that only marks an error field in red, with no icon or text, is invisible to a meaningful share of users. Always pair color with a second signal — an icon, an underline, explicit text.
Not every user can use a mouse or touchscreen. Every interactive element needs a visible focus state (covered in the earlier feedback/states lesson) so someone navigating by keyboard alone can see exactly where they are — and every action reachable by click needs to be reachable by keyboard too.
Every meaningful image needs alt text describing what it conveys, for screen reader users. Every form field needs a real, programmatically-associated label — not just a visually nearby placeholder that a screen reader can't reliably connect to the field.
Build it in from the start, not after
Accessibility is not a final polish step to add if time allows — retrofitting it after a design is finished is far more expensive than building it in from the start, the same way it's far cheaper to fix a structural wireframe problem than a finished visual design.