What does digital accessibility actually require?
That people with disabilities can perceive, operate, understand and use a digital service — and the practical standard almost everywhere is WCAG, whose four principles are exactly those four words.
The four principles, with what they actually mean:
Perceivable. Information must be available to the senses a user has. Text alternatives for images, captions and transcripts for media, sufficient colour contrast, and never conveying information by colour alone.
Operable. Everything must work without a mouse. Full keyboard operability is the single highest-value requirement, because it also serves switch devices, voice control and screen readers. No keyboard traps, visible focus indicators, adequate time limits, and no content that flashes at rates that can trigger seizures.
Understandable. Predictable navigation, clear language, labelled form fields, and error messages that identify the problem and how to fix it.
Robust. Works with assistive technology — which in practice means using correct semantic HTML rather than recreating controls from generic elements. A div styled as a button is invisible as a button to a screen reader.
The conformance levels: A, AA and AAA. AA is the level referenced by most legislation and is the practical target; AAA is not expected across a whole site.
Who it is legally required of. Public sector bodies in many jurisdictions are under explicit statutory duties, including publishing an accessibility statement. Private organisations are covered by general equality and disability discrimination law, which requires reasonable adjustments — and litigation in this area has grown substantially.
What is most commonly wrong, and this list covers a large share of real failures:
Images without meaningful alternative text, and decorative images that should be marked as such and are not.
Form fields without labels.
Poor contrast, particularly grey text on white.
Custom components built without keyboard support or accessible names.
Missing document structure — no headings, or headings used for visual size.
Videos without captions.
Automated testing catches perhaps a third of issues. Keyboard-only testing and testing with real users are what actually find the rest.