The principles of responsive web design for mobile devices include fluid grids, which use relative units to proportionally resize elements, allowing for a seamless viewing experience across various screen sizes. Flexible images that automatically scale to fit the screen are vital to avoid distortion, while media queries adjust the layout and style based on the device’s characteristics, such as width and height. Additionally, prioritizing user experience ensures that content is easily accessible and navigable regardless of the device used. By adhering to these principles, web designers can create sites that perform effectively on mobile platforms.