One of the main principles of responsive web design is flexibility. This means that the website design should be flexible and adapt to different screen sizes, which is achieved by using relative measurements instead of absolute measurements. That is, instead of setting exact width and height sizes for elements such as images, percentages or relative units of measurement, such as “em” or “rem”, are used to make elements automatically adjust to the screen size.
Another important principle is simplicity. In a responsive website, simplicity is key. It is about creating a minimalist and efficient design that allows users to access the information they need quickly and easily. This is achieved by using simple and clean elements, with a limited color palette and a clear and uncluttered layout.

Usability is another fundamental principle of responsive web design. Responsive websites should be easy to use and navigate, regardless of the device used. This is achieved by optimizing navigation, organization, and presentation of content to suit different screen sizes. It is important to note that usability does not only refer to navigation, but also to the website's loading speed and the responsiveness of the design.
Accessibility is another key principle of responsive web design. A responsive website should be accessible to all users, regardless of their physical or technical ability. This means that the design should be easy to read and navigate, with elements that are clear and visible. It also means that the website should be compatible with different browsers and operating systems, so that users can access the website from any device they use.
Mobile-friendliness is another key principle of responsive web design. More and more users are accessing websites from their mobile devices these days. For this reason, it is essential for a website to be mobile-friendly and adapt to different screen sizes. This involves creating a design that is easy to read and navigate on small screens, and optimizing the website's loading speed for mobile devices.
In short, responsive web design principles focus on creating a website that is flexible, simple, usable, accessible, and mobile-friendly. By following these principles, you can create a website that offers an optimal user experience on any device, which can help increase traffic, improve conversion rates, and lead to higher user satisfaction.