Contrast is the difference in luminance between text and its background, expressed as a ratio. The widely used web accessibility guidelines set a minimum of 4.5 to 1 for normal text and 3 to 1 for large text, meaning roughly 18-point regular or 14-point bold, and for interface components such as input borders and focus indicators. Light gray hint text on white, pale blue links, and white text on a light photograph all fail routinely. You do not have to guess: a free contrast checker takes two colors and gives you the ratio. Check it when you choose a palette, not after the flyer is printed.
Color is a fine way to emphasize meaning and a poor way to carry it. About one in twelve men and a smaller share of women have some form of color vision deficiency, screen readers do not announce color at all, and printed copies are often black and white. So a required field needs the word “required” or an asterisk that is explained; an error needs text that says what went wrong, next to the field; a status needs a label or an icon, not just red, amber and green; a chart needs patterns, direct labels or distinct shapes as well as color.
Keyboard operability is the test that catches the most serious interaction barriers. Many people cannot use a mouse: people who are blind, people with tremor, pain or limited hand mobility, people using a switch or voice control that sends key presses. Every control must be reachable with Tab, activated with Enter or Space, and, for groups such as radio buttons and menus, navigated with arrow keys. The focus indicator, the outline that shows which element is active, must be visible; removing it for a cleaner look removes the only map a keyboard user has. There must be no keyboard trap, a place where focus enters and cannot leave. And any interaction that depends on dragging, hovering or drawing needs an equivalent that uses key presses. Drag-and-drop sorting, image hotspots and sliders are the usual offenders in learning content; each can be built with a keyboard alternative or replaced with an equivalent activity.
