CSS pseudo-classes such as :valid, :invalid, :required, and :optional are powerful tools for styling form fields based on their validation state. By targeting these pseudo-classes in your CSS, you can visually highlight errors or confirm correct input as users interact with your forms. This immediate feedback helps guide users to complete forms accurately and efficiently. For example, you can use :invalid to highlight fields with incorrect input in red, or :valid to show a green border when the field content meets the requirements. The :required pseudo-class is useful for marking fields that must be filled, while :optional can help you identify fields that are not mandatory.

Providing visual feedback as users fill out your form helps them quickly spot and fix mistakes. Use clear color contrasts, such as green for valid and red for invalid states, to make feedback noticeable. However, do not rely on color alone—combine color with icons or text messages for users who may have difficulty distinguishing colors. For accessibility, always ensure that feedback is clear for all users, including those using assistive technology.