Typically we write one codebase to support all these different device types. 

The problem with normal CSS is, for example, an image could be 500 pixels wide.. but what if a viewer with a phone only has a 425 pixel-wide device? Now your website looks broken. That's no good. 

Responsive web design is the act of making your HTML elements "snap" to different locations and to look different on different devices. 

In this course, you'll use your knowledge of background colors, border radii, font colors, flexbox, and grid to start making a web page act responsively. 

Lastly, you'll finish this course by creating a page layout using flexbox (alternatively you can use CSS grid) and making the template responsive – that is, you'll make it "mobile-friendly" so when you shrink down your browser size, it looks great on tablets and phones.