The Role of HTML
The marked up HTML document is said to be the structural layer of a web page. It is the foundation upon which the presentation layer (instructions for how the elements should be delivered or displayed) and the behavioral layer (scripting and interactivity) are applied.
Did you happen to read the preceding XML chapter? It may seem off the topic of HTML, but there are some critical XML-based concepts there that guide the way HTML is perceived and handled in contemporary web design. One guiding concept is that the fundamental purpose of HTML as a markup language is to provide a semantic description (the meaning) of the content and establish a document structure. It is not concerned with presentation, such as how the document will look in a browser. Presentation is the job of Cascading Style Sheets, which is covered in Part III.
That presentational instructions should be kept separate from the semantic and structural markup is nothing new. It has been the intent of HTML from its beginning as an application of SGML (Standardized General Markup Language) as noted in the upcoming sidebar. What is new is that the web community is recognizing that there are measurable advantages (in terms of time and money) to using HTML for what it was designed to do, and nothing more.
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