The three basic requirements for responsive web design are:
Fluid Grids: This essentially means that the layout of the website uses relative units like percentages, instead of absolute units like pixels, for page element sizing. This enables the layout to adjust to different screen sizes smoothly.
Flexible Images: Images on the site must also be flexible to adapt to the layout changes caused by different screen sizes. This means the images can scale and resize to fit within the confines of the layout without distortion or loss of clarity.
Media Queries: Part of CSS3, media queries allow the content to adapt to different conditions, such as device screen size. They can be used to apply different styles for different devices, providing optimal viewing experiences across a wide range of devices.