Web design with HTML5 and CSS3 HTML5, which is the fifth release for the Hypertext Markup Language, is the major markup language used to define and structure content on web pages. All web pages in existence have implemented HTML in one way or another. CSS, which stands for Cascading Style Sheets, revolutionized how to define the layout of webpages, most of which were built on HTML. CSS3 is its third and latest release. CSS file contains definitions for different web page elements such as headers, footers, sidebars, links, menus and even text.