A website's User Interface ("UI") has two components. First, there are the visual elements, such as colors, fonts, and images. Second, there is the placement or positioning of those elements. In Responsive Web Design, a UI layout must accommodate many different browsers and devices accessing the content.

CSS3 introduced Flexible Boxes, or flexbox, to create page layouts for a dynamic UI. It is a layout mode that arranges elements in a predictable way for different screen sizes and browsers. While somewhat new, all popular modern browsers support flexbox. This section covers how to use flexbox and the different layout options it offers.