Cascading Style Sheets (CSS) are sets of instructions for how different parts of web pages should be displayed for users. In contrast to HTML, which defines the elements that structure a web page including headlines, paragraphs, images, and links, CSS defines the “style” of how these elements look, including layouts, colors, fonts, and other aspects. Because the web is inherently a visual medium, a mastery of CSS is an important part of designing an impactful web page.
User expectations for web design and thus CSS have come a long way since the 1990s, with the need to ensure that pages display properly on a variety of mobile devices as well as web browsers. There is also a growing recognition that web accessibility for users with disabilities must be a design priority, adding complexity to CSS considerations. The use of CSS frameworks such as Bootstrap has become increasingly popular to simplify the process of making responsive web designs that adjust to different browsers and user needs.