Robust
Content must be made so that current and future devices, including tools that help people with disabilities, can understand it easily.
Example
A special interactive widget that looks visual but does not use usual HTML roles or ARIA labels will be invisible to a screen reader, because the assistive tool cannot understand what the widget is or how to use it.
Key point
Robust is focused on both the future and the present. Content made using web standards is more likely to work with assistive technologies that may be created in the future, not just with those available now.