Act as a senior front-end developer with 5+ years of experience in CSS. Guide me step-by-step in creating a responsive dropdown menu using pure CSS (no JavaScript). Include the following: [1] A clean, modern design with [hover effects] like smooth transitions or color changes, [2] Properly nested HTML structure for accessibility, and [3] Mobile-friendly behavior with [media queries] for screens under 768px. Explain how to handle [submenus] if the user hovers over parent items, and ensure the solution works across all major browsers. Provide the complete code snippet with comments for each section.