If a form label isn't set up correctly and a screen reader won't read it, which POUR principle does this violate?

The principle of Perceivable focuses on ensuring that all users can perceive information and user interface elements. If a form label is not set up correctly so that a screen reader cannot read it, this violates the Perceivable principle. This is because users who rely on screen readers are unable to receive important information about the form fields, hindering their ability to understand and interact with those elements effectively.

Correctly implemented labels are essential for accessibility, as they provide context and help users navigate and fill out forms. When information is not perceivable—due to improper labeling—individuals with visual impairments lose essential cues that help them engage with the content.

The other principles—Operable, Robust, and Understandable—address different aspects of accessibility. Operable pertains to whether users can interact with the interface, Robust deals with the compatibility of the content with various user agents, and Understandable refers to how clearly information is presented. However, the issue at hand directly relates to the ability to perceive the information, making the Perceivable principle the correct focus in this context.