Except for the <html>, <head>, <body>, and <title> tags, the HTML standard has few other required structural elements. You're free to include pretty much anything else in the contents of your document. (The Web surfers among you know that HTML authors have taken full advantage of that freedom, too.) Perhaps surprisingly, though, there are only three main types of HTML content: tags (which we describe above), comments, and text.
Like computer-programming source code, a raw HTML document, with all its embedded tags, can quickly become nearly unreadable. We strongly encourage that you use HTML comments to guide your composing eye.
Although it's part of your document, nothing in a comment, including the body of your comment that goes between the special starting tag "<!- -" and ending tag delimiters "- ->" gets included in the browser display of your document. Now you see a comment in the source, like in our simple HTML example, and now you don't on the display, as evidenced by our comment's absence in Figure 2.1. Anyone can download the source text of the HTML document and read the comments, though, so be careful what you write. [the section called "Comments"]
If it isn't a tag or a comment, it's text. The bulk of content in most of your HTML documents--the part readers see on their browser displays--is text. Special tags give the text structure, such as headings, lists, and tables. Others advise the browser how the content should be formatted and displayed.
What about images and other multimedia elements we see and hear as part of our web browser displays? Aren't they part of the HTML document? No. The data that comprise digital images, movies, sounds, and other multimedia elements that may be included in the browser display are in documents separate from the HTML document. You include references to those multimedia elements via special tags in the HTML document. The browser uses the references to load and integrate other types of documents with your HTML text.
We didn't include any special multimedia references in the previous example simply because they are separate, nontext documents you can't just type into a text processor. We do, however, talk about and give examples on how to integrate images and other multimedia in your HTML documents later in this chapter, as well as in extensive detail in subsequent chapters.