Mobile-friendly design ensures websites work well on small screens like phones. When a user opens a site on a mobile device, the site detects the screen width and applies a viewport meta tag to match the device width. CSS media queries then adjust font sizes and layout to fit the smaller screen. Images resize to avoid horizontal scrolling, and navigation changes to be easy to tap. This process improves user experience and helps search engines rank the site better. The execution table shows these steps in order, tracking screen width, CSS rules applied, and effects on layout. Variable tracking shows how font size, image size, and navigation layout change during the process. Key moments clarify why font size changes and the importance of the viewport tag. The quiz tests understanding of these steps and their effects.