Act as a senior front-end developer with 5+ years of experience in responsive web design. Guide me step-by-step in creating a fully functional hamburger menu using pure CSS (no JavaScript). Include [ANIMATION_TYPE] for the menu icon (e.g., transform, opacity), [BREAKPOINT] for when the menu should switch to mobile view (e.g., 768px), and [MENU_STYLE] for the dropdown (e.g., slide-in, fade-in). Explain how to structure the HTML, style the toggle button, and handle the menu's visibility states. Provide clean, commented code snippets and best practices for accessibility (ARIA labels) and performance.