Responsive design is fundamentally about using Fluid Grids, which allow elements on a webpage to resize proportionally rather than with fixed dimensions. This makes the design adaptable to various screen sizes. The Viewport meta tag is also crucial, as it controls the layout on mobile browsers by specifying the visible area of a web page.

Media Queries play a pivotal role in responsive design. They allow the application of different CSS styles based on the device's characteristics, such as screen width, height, and orientation. Flexbox is another essential tool, providing a layout structure that can adjust automatically based on the screen size. Responsive Images ensure that the appropriate image size is loaded based on the device’s screen size and resolution, optimizing both performance and user experience.

Mobile-First Design is a strategy where the design process starts with the smallest screen and scales up. This ensures that the essential content is always prioritized. Breakpoints are defined in CSS to apply specific styles at certain screen widths, allowing for a smooth transition between different layouts. Adaptive Design is another approach, where different layouts are created for different devices, unlike the single, fluid layout in responsive design.

Responsive design greatly enhances User Experience (UX) by ensuring that websites are accessible and usable on all devices. Mobile Optimization is a critical aspect, particularly for Mobile Email Marketing and Call to Action (CTA) elements, which need to be clearly visible and easily clickable on smaller screens. Contextual Marketing benefits from responsive design by delivering personalized content that fits well on any device, improving engagement and conversion rates. Finally, Landing Pages designed responsively ensure that users have a consistent experience, which is crucial for Cross-Browser and Cross-Device Compatibility.