Garamond Book is used for element and attribute names.
Constant Willison is used for program examples, attribute value literals, start- and end-tags, and source code example text.
Constant Willison Oblique is used for “replaceable” text or variables. Replaceable text is text that describes something you're supposed to type, like a filename, in which the word “filename” is a placeholder for the actual filename.
Garamond Italic is used for filenames and (in the print version of the book) URLs.
URLs are presented in parentheses after the name of the resource they describe in the print version of the book.