Throughout the book, we use a constant-width typeface to highlight any literal element of the HTML/XHTML standards, tags, and attributes. We always use lowercase letters for tags. We use italic for filenames and URLs and to indicate new concepts when they are defined. Elements you need to supply when creating your own documents, such as tag attributes or user-defined strings, appear in constant-width italic in the code.
 HTML is case-insensitive with regard to tag and attribute names, but XHTML is case-sensitive. And some HTML items, such as source filenames, are case-sensitive, so be careful.
We discuss elements of the language throughout the book, but you'll find each one covered in depth (some might say in nauseating detail) in a shorthand, quick-reference definition box that looks like the one that follows. The first line of the box contains the element name, followed by a brief description of its function. Next, we list the various attributes, if any, of the element: those things that you may or must specify as part of the element.
We use the following symbols to identify tags and attributes that are not in the HTML 4.01 or XHTML 1.0 standards but are additions to the languages:
The description also includes the ending tag, if any, for the element, along with a general indication of whether the end tag may be safely omitted in general use in HTML. For the few tags that require end tags in XHTML but do not have them in HTML, the language lets you indicate that by placing a forward slash (/) before the tag's closing bracket, as in <br />. In these cases, the tag may also contain attributes, indicated with an intervening ellipsis, such as <br ... />.
The "Contains" header names the rule in the HTML grammar that defines the elements to be placed within this tag. Similarly, the "Used in" header lists those rules that allow this tag as part of their content. These rules are defined in Appendix A.
Finally, HTML and XHTML are fairly intertwined languages. You will occasionally use elements in different ways depending on context, and many elements share identical attributes. Wherever possible, we place a cross-reference in the text that leads you to a related discussion elsewhere in the book. These cross-references, like the one at the end of this paragraph, serve as a crude paper model of hypertext documentation, one that would be replaced with a true hypertext link should this book be delivered in an electronic format. [Section 3.3.1]
We encourage you to follow these references whenever possible. Often, we cover an attribute briefly and expect you to jump to the cross-reference for a more detailed discussion. In other cases, following the link takes you to alternative uses of the element under discussion or to style and usage suggestions that relate to the current element.