Valid XML Document - Key Terminology

Key Terminology

The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs that appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use.

(Unicode) character
By definition, an XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.
Processor and application
The processor analyzes the markup and passes structured information to an application. The specification places requirements on what an XML processor must do and not do, but the application is outside its scope. The processor (as the specification calls it) is often referred to colloquially as an XML parser.
Markup and content
The characters making up an XML document are divided into markup and content, which may be distinguished by the application of simple syntactic rules. Generally, strings that constitute markup either begin with the character < and end with a >, or they begin with the character & and end with a ;. Strings of characters that are not markup are content. However, in a CDATA section, the delimiters are classified as markup, while the text between them is classified as content. In addition, whitespace before and after the outermost element is classified as markup.
A markup construct that begins with < and ends with >. Tags come in three flavors:
  • start-tags; for example:
  • end-tags; for example:
  • empty-element tags; for example:
A logical document component which either begins with a start-tag and ends with a matching end-tag or consists only of an empty-element tag. The characters between the start- and end-tags, if any, are the element's content, and may contain markup, including other elements, which are called child elements. An example of an element is Hello, world. (see hello world). Another is .
A markup construct consisting of a name/value pair that exists within a start-tag or empty-element tag. In the example (below) the element img has two attributes, src and alt:
Foligno Madonna, by Raphael Another example would be
Connect A to B. where the name of the attribute is "number" and the value is "3".
XML declaration
XML documents may begin by declaring some information about themselves, as in the following example:

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