Displaying error messages inline—directly beside the form field where a user has made a mistake—significantly improves user experience. Inline messages provide immediate, contextual feedback, allowing you to quickly identify which field needs attention and what action is required. This approach is much less disruptive than using alert popups, which interrupt your workflow and can cause frustration. Structuring inline error messages in HTML typically involves placing a dedicated container, like a span or div, next to each input field. You can either include these containers in your markup from the start, or dynamically create and insert them as needed during validation.

To ensure your inline error messages are effective, clarity and accessibility are essential. Always use simple, direct language that tells the user exactly what is wrong and how to fix it. Avoid vague statements like Invalid input; instead, specify the issue, such as Password must be at least 8 characters. Place error messages close to the relevant input field so users do not have to search for feedback. For accessibility, make sure error messages are programmatically associated with their fields using aria-describedby or similar attributes, and use sufficient color contrast for readability. Remember to remove error messages as soon as the user corrects the input, providing positive feedback when possible.