What are the core concepts of responsive web design?

The core concepts of responsive web design are fluid grids, flexible images, and media queries.

Responsive web design is a web development approach that creates dynamic changes to the appearance of a website, depending on the screen size and orientation of the device being used to view it. It is about making websites adaptable to the device on which they are being viewed, providing an optimal user experience.

The first core concept, fluid grids, is about using relative units like percentages, instead of absolute units like pixels, for all layout components. This allows the layout to resize in relation to the screen size. For example, if you have a container that is 960px wide on a 1024px wide screen, it would be 94% of the screen width. On a smaller screen, say 480px wide, the container would still be 94% of the screen width, but now it would be 451.2px wide. This fluidity provides a consistent layout across different screen sizes.

The second core concept, flexible images, is also known as adaptive images. This concept ensures that images on a website are flexible enough to fit different screen sizes. This is achieved by setting the max-width property of an image to 100% of its container. This means that an image will never be larger than its container, allowing it to scale down on smaller screens.

The third core concept, media queries, is a CSS technique used to apply different style rules to different devices based on their characteristics, primarily screen size and resolution. Media queries allow designers to build multiple layouts within one website, each optimised for a different screen size. For example, a media query could specify that if a screen is less than 600px wide, the website's layout changes to a single column layout instead of a multi-column layout.

In conclusion, responsive web design is about ensuring that a website looks good and functions well on any device. It achieves this through the use of fluid grids, flexible images, and media queries. These core concepts work together to create a seamless user experience, regardless of the device used to access the website.