An item that communicates to a screen reader that it is expanded or collapsed is an example of which web accessibility principle?

The concept of an item communicating to a screen reader that it is expanded or collapsed relates to the web accessibility principle of being "robust." This principle emphasizes that content must be compatible with a wide range of user agents, including assistive technologies like screen readers. When a user interface component can inform assistive technology about its state—expanded or collapsed—it ensures that content remains accessible regardless of the user's choice of tools.

Robustness allows for a diverse array of technologies to properly interpret and use the web content, ensuring that all users, including those who rely on screen readers, can understand the structure and functionality of web elements. This ensures a higher level of accessibility and interoperability across different browsing environments.

The other principles—understandable, operable, and perceivable—also play important roles in accessibility but focus on different aspects. Being understandable involves ensuring that information is clear and straightforward, operable focuses on user input and navigation, and perceivable pertains to making content available through various senses. While these elements contribute to a fully accessible experience, the specific action of communicating state changes aligns most closely with the robustness principle.