Responsive web design is an approach to designing and developing websites that adapt and respond to various screen sizes and devices. It ensures that web pages look and function well on different devices, including desktops, laptops, tablets, and smartphones. CSS plays a crucial role in implementing responsive design principles. Here are some key CSS techniques for creating responsive web designs:

1. Media Queries: Media queries allow you to apply different CSS styles based on the characteristics of the device or viewport size. By specifying different CSS rules for specific screen widths or device features, you can adjust the layout and appearance of your web page. Here’s an example:

@media (max-width: 768px) {
  /* CSS rules for screens up to 768px wide */
}

@media (min-width: 769px) and (max-width: 1024px) {
  /* CSS rules for screens between 769px and 1024px wide */
}

@media (min-width: 1025px) {
  /* CSS rules for screens wider than 1024px */
}

1. Fluid Grids: Using CSS grids or flexbox, you can create fluid layouts that adapt to different screen sizes. By specifying relative units like percentages or fr (fractional units), the layout adjusts proportionally based on the available space.
2. Flexible Images: To ensure images scale and resize appropriately, use CSS properties like max-width: 100% to make images responsive and prevent them from overflowing their containers.
3. Responsive Typography: Use relative units like em, rem, or vw (viewport width) for font sizes to ensure that text scales properly on different devices.
4. CSS Flexbox and Grid: CSS flexbox and grid layout modules offer powerful tools for building responsive designs. They provide flexible and dynamic options for arranging and aligning elements within containers.
5. Hidden and Shown Elements: CSS can be used to hide or show specific elements based on screen size or other conditions. This can help optimize the content and user experience for different devices.
6. Mobile-First Design: Adopting a mobile-first approach involves designing and developing for mobile devices first, then progressively enhancing the layout and features for larger screens. This approach helps ensure that your design works well on smaller screens and gracefully expands for larger ones.

Remember, responsive web design is not just about adjusting the layout; it also involves considering touch interactions, optimizing performance, and providing a seamless user experience across different devices.

By leveraging CSS techniques like media queries, fluid grids, flexible images, responsive typography, and the power of CSS flexbox and grid, you can create responsive web designs that adapt beautifully to various screen sizes and devices.