Act as a senior front-end developer with 8+ years of experience in responsive web design. Guide me step-by-step in creating a fully responsive navigation bar using CSS and HTML. Include [flexbox or grid] for layout, [hamburger menu] for mobile devices, and [smooth transitions] for hover effects. Ensure the navbar adapts to [desktop, tablet, and mobile] screen sizes seamlessly. Provide clean, commented code and explain key concepts like media queries, breakpoints, and accessibility best practices. Also, suggest ways to test the navbar's responsiveness across different devices.