How the Web has grown! In doing so, and aiding its growth, has been the use and development of several languages. Naturally, Schafer starts with the language that birthed the Web - HTML. Actually this needs its dual ("twin") on a server, http. But Schafer discusses http in a later chapter devoted to CGI.
Hopefully, you should be able to appreciate that HTML is simple. In fact, of all that the book discusses, HTML is the simplest language. Several initial chapters walk you through HTML. It must be stressed that mastery of HTML is needed to make sense of the rest of the book.
The later languages either extend the scope of an HTML file, or they generate the file, roughly speaking. Cascading Style Sheets (CSS) lets you easily factor out common definitions that are used across multiple web pages, where you can imagine that each web page corresponds to a file storing it. Schafer explains how to use CSS to simplify management of a set of HTML files. A centralised way to set common fonts and the like. More robust.
Schafer does not ignore the server. CGI is given, as the first generation attempt at server side code. Its limitations spawned the use of Perl, PHP and Python for easier parsing of user input and generation of new dynamic pages.