Discuss the principles of responsive web design and the techniques used to ensure that web applications provide an optimal user experience across different devices.

Responsive web design principles include fluid grids, flexible images, and media queries. Techniques to ensure optimal user experience across devices involve using relative units for layout elements, applying CSS media queries to adjust styles based on screen size and resolution, and employing a mobile-first approach to design. These practices ensure that web applications adapt to various screen sizes and orientations, providing a consistent and user-friendly experience on desktops, tablets, and mobile devices.