XML in a Nutshell, 3rd Edition

Tag Syntax

Superficially, XML tags look like HTML tags. Start-tags begin with < and end-tags begin with </. Both of these are followed by the name of the element and are closed by >. However, unlike HTML tags, you are allowed to make up new XML tags as you go along. To describe a person, use <person> and </person> tags. To describe a calendar, use <calendar> and </calendar> tags. The names of the tags generally reflect the type of content inside the element, not how that content will be formatted.

Empty elements

There’s also a special syntax for empty elements, elements that have no content. Such an element can be represented by a single empty-element tag that begins with < but ends with />. For instance, in XHTML, an XMLized reformulation of standard HTML, the line-break and horizontal-rule elements are written as <br /> and <hr /> instead of <br> and <hr>. These are exactly equivalent to <br></br> and <hr></hr>, however. Which form you use for empty elements is completely up to you. However, what you cannot do in XML and XHTML (unlike HTML) is use only the start-tag—for instance <br> or <hr>—without using the matching end-tag. That would be a well-formedness error.

Case-sensitivity

XML, unlike HTML, is case-sensitive. <Person> is not the same as <PERSON> or <person>. If you open an element with a <person> tag, you can’t close it with a </PERSON> tag. You’re free to use upper- or lowercase or both as you choose. You just have to be consistent within any one element.