Understandable

Getting things visible and operable still leaves a product unusable if nobody can tell what is going on. The classic case is a signup screen where an input's border turns red and no text appears. For someone who can't separate the colors, nothing on screen changed at all — and even for someone who sees the red, what was wrong and why is still unstated.

A page that never declares its language produces the same failure. A screen reader picks its pronunciation rules from that value, so with the declaration missing it reads Korean sentences with English pronunciation. Every character arrives; none of the meaning does.

So alongside can you see it and can you operate it, the guidelines made can you make sense of it a principle of its own.

The criteria under this principle group into three.

1. Making things readable. Declare the language of the document and of any passage that switches, and define terms and abbreviations that only mean something inside this product. Ship internal shorthand onto a screen and newcomers and assistive-technology users get stuck together.
2. Making things predictable. Keep a given function in the same place on every screen, and don't let merely selecting something change the screen. When picking an item in a list jumps straight to the next screen, anyone stepping through with a keyboard changes screens just by passing over it.
3. Helping with input. Say in words what a field is for, and when validation fails, state which field failed and why. One line saying the password needs at least eight characters does what a red border cannot. For submissions that can't be undone, provide a review step or a way to reverse them. The two are interchangeable, but one of them has to exist.

Checking it is simple enough, and the same test catches most of these criteria in one pass. Strip the color and the shapes off the screen, leave only the text, and see whether what went wrong and what to do about it still comes through. If it doesn't, that information was riding on color and position alone.

"The border turns red with no message, so this fails on understandable."

The information is carried by color only.

"The page language declaration is missing. Screen readers are reading it as English."

A readability criterion is absent.

"Selecting in the dropdown shouldn't navigate. Let's add a confirm button."

Protecting the predictability criteria.

Pagedeck's signup screen failed on this. A password that didn't meet the rules only turned the border red. Noa wrote the rules out under the field and made the remaining conditions visible while typing. Drop-off caused by the password field halved — and that fix was not only for users who can't distinguish colors.