Understandable
Users need to understand both the content and how the interface works.
Key point
Understandable is the most human of the four principles. It means thinking about how people think, how they use language, and what they expect, not just about technical details.
Readable
The text must be easy to read and understand. You should also say what language the page is in so that assistive tools can pronounce words correctly.
Example
A screen reader set to English will say French words wrong unless the HTML says the passage is in French. This makes the user hear confusing audio that does not show what the text means.
Predictable
Pages and parts must work in ways that users expect, without sudden or confusing changes.
Example
If picking an option from a dropdown menu automatically sends a form and opens a new page, a user who is looking through choices by pressing arrow keys might cause something to happen by mistake and cannot easily fix it.
Input assistance
When users make mistakes in forms, the interface should help them find and fix those mistakes.
Example
A checkout form that only makes a wrong field border red does not tell a screen reader user what is wrong or how to fix it. A clear error message in words, connected to the right field, helps all users.