Operable

Content arriving and a function working are two different problems, and the second one doesn't follow from the first. A screen that can be read but not pressed still can't be used.

Pagedeck's sharing screen was in exactly that state. Every image had a description, and the brightness difference between text and background cleared the threshold. But the dropdown for choosing permissions opened only with a mouse. Somebody working from the keyboard could read as far as the document title and never finish sharing. Everything on the perception side passed, and there was nothing you could actually do with the screen.

That's why the Web Content Accessibility Guidelines (WCAG), the international standard for web accessibility, pulled operability out as its own when it divided the rules into four principles. Unlike robust, which asks whether things reach machines correctly, or perceivable, which asks whether information can be taken in through the senses, this principle asks only whether a person can put their hands on it and get to the end.

Under the principle sit statements you can test as true or false. Four of them come up most in practice.

1. Every function has to be reachable without a mouse. Building the path — tab to move, enter to activate, arrow keys through a list — is what keyboard navigation means. Hand-built dropdowns and modals are where it's most often missing.
2. The currently selected position has to be visible. That marker is the focus ring, and deleting it because it looks untidy fails one criterion on the spot. Somebody working by keyboard is then pressing enter without knowing where they stand.
3. A screen with a time limit has to offer a way to extend it. If the session logs out automatically after 20 minutes, announce the time remaining and put an extend button there.
4. Nothing may flash more than three times per second. Rapid flashing can trigger photosensitive seizures, which is why the criterion forbids it.

The requirement to keep tap areas above a minimum size lives under this principle too. Even where the icon artwork is only 16 across, the pressable area around it gets set to something near 44.

Checking it needs no tools. Take your hand off the mouse and go through one whole flow with the tab key. A single button you can't reach anywhere in that flow is one criterion failed.

"This dropdown won't open with tab. That's an operable criterion failing."

It's visible on the screen, and there's no path by which anybody can operate it.

"Don't delete the focus ring. Deleting it fails a criterion immediately."

Saying the standard outranks visual tidiness.

"If it logs out after 20 minutes, there has to be an extend button."

The means of satisfying the time-limit criterion is missing.

At Pagedeck, Theo goes through every new screen once without touching the mouse. Noa draws the focus marker into the mockups in advance.