This principle covers readable text (including identifying the page’s language in code so screen readers use correct pronunciation), predictable behavior (a page shouldn’t unexpectedly change context when a user simply focuses on a field), and input assistance, meaning forms have clear labels, instructions, and specific, actionable error messages rather than a generic “invalid input.”
Understandable failures disproportionately affect users with cognitive, learning, and language-processing disabilities, even though they’re sometimes overlooked in audits that focus mainly on screen reader compatibility.