The document discusses responsive design using HTML5 and CSS3. It defines the problem of creating separate style sheets for different mobile screen widths. The current solution uses a device recognition system to select the appropriate CSS file. The proposed solution is responsive design, which uses a flexible grid, flexible images, and media queries to dynamically adapt the layout for different screen sizes using only one CSS file. Responsive design allows the page to automatically resize and re-arrange elements to fit different screen widths.