HTML and CSS are core technologies through which information encoded in a way that can be understood by web browsers. They have distinct functions that should be respected for maximum portability, clarity, and easiest maintenance.
This chapter describes the function of HTML as a means to deliver information over the Web. In this introduction, no clear distinction is made between the semantic function of HTML and the presentational function of CSS. You will be learning more about this in sub-subunit 1.3.2 What are Cascading Style Sheets (CSS)? For now, just keep in mind that this is because some websites - especially those that are very old, or those managed by certain content management systems - rely on HTML to markup both semantic content (that is: what content means) and presentational content (that is: how specific content should be displayed). In the best-practice coding that we will be practicing, we will abstract as much presentational information out of HTML files by putting them into external CSS files.