Except for the <html>, <head>, <body>, and <title> tags, the HTML and XHTML standards have 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 authors have taken full advantage of that freedom, too.) Perhaps surprisingly, though, there are only three main types of HTML/XHTML content: tags (which we described previously), comments, and text.
A raw document with all its embedded tags can quickly become nearly unreadable, like computer-programming source code. We strongly recommend that you use comments to guide your composing eye.
Although it's part of your document, nothing in a comment, which goes between the special starting tag <!-- and ending tag --> comment delimiters, gets included in the browser display of your document. You see a comment in the source, as in our simple HTML example, but you don't see it on the display, as evidenced by our comment's absence in Figure 2-1. Anyone can download the source text of your documents and read the comments, though, so be careful what you write. [Section 3.5.3]
If it isn't a tag or a comment, it's text. The bulk of content in most of your HTML/XHTML 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 comprises digital images, movies, sounds, and other multimedia elements that may be included in the browser display are in documents separate from the main HTML/XHTML document. You include references to those multimedia elements via special tags. The browser uses the references to load and integrate other types of documents with your text.
We didn't include any special multimedia references in the previous example simply because they are separate, nontext documents that you can't just type into a text processor. We do, however, talk about and give examples of how to integrate images and other multimedia in your documents later in this chapter, as well as in extensive detail in subsequent chapters.