Implement a simple web app that exercises core frontend skills: 1) Implement a list view that renders items from an in-memory array. Use exact CSS class names with double underscores (e.g., list__item); a single underscore must not satisfy tests. 2) Add a submission form to create a new list item. On submission failure (e.g., simulated 400), do not clear form fields or lose user input; show a validation message. On success, append to the list and clear the form. 3) Fetch list data from a backend API endpoint and render it, handling loading, success, and error states; avoid duplicate entries when reconciling server data with locally added items. 4) Ensure accessibility (labels tied to inputs, correct button types), idempotent updates, and predictable re-renders. Explain your event handling, state management, and error-handling strategies, plus how you would structure files/components and test these behaviors.