Core Concepts of Responsive Web Design:

Responsive web design is crucial in the development of websites that function well on a variety of devices and window or screen sizes. It primarily involves using HTML and CSS to resize, hide, shrink, enlarge, or move the content to make it look good on any screen.

1. Fluid Grids: The use of fluid grids is one foundational principle. Unlike fixed-width grids, which use pixels, fluid grids use relative units like percentages. This flexibility helps ensure that the layout adjusts smoothly across screens.

2. Flexible Images: Responsive websites also need images that can adapt to various layouts. Using CSS, you can set max-width to 100% to ensure images resize within a container. This prevents images from disrupting the layout on smaller screens.

3. Media Queries: Media queries allow developers to create multiple layouts using the same HTML document by applying different CSS rules based on device characteristics such as width, height, and resolution. This technique is essential for applying different styles to different devices.

Keeping these elements in mind will ensure a website offers a smooth user experience regardless of device.