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What does “XML tag set” mean?
“XML tag set” is an informal term for an XML vocabulary: the element and attribute names available to a particular document type or application, plus the conventions for using those names. The W3C’s XHTML 2.0 glossary describes a vocabulary as a collection of element and attribute names, and distinguishes it from the grammar that prescribes their use (W3C XHTML 2.0 glossary).
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For example, a book-catalog application might use book, title, and author. Those names belong to that application’s vocabulary; they are not universal XML keywords.
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Does XML have a fixed set of tags?
No. XML defines the syntax for writing markup, but it does not dictate the application-level names or meanings of elements and attributes. The W3C XML 1.0 specification states: “This specification does not constrain the application semantics, use, or (beyond syntax) names of the element types and attributes,” with an exception reserving names beginning with any case combination of “XML” for standardization (W3C XML 1.0, Fifth Edition).
That means two XML applications can use different vocabularies while following the same XML syntax. The name title, for instance, has no built-in meaning to XML; an application or document definition determines what it represents.
How tags, elements, and names differ
- Tag: The markup written in angle brackets. A start-tag begins a non-empty element, an end-tag closes it, and an empty-element tag expresses an element without a separate end-tag.
- Element type name: The name carried in a tag, such as
bookortitle. - Element: The logical structure identified by its tags. It can contain text, child elements, or both, depending on the document’s rules.
- Attribute: A name-value detail written in a start-tag, such as
id="b17".
For example, in <title>Example</title>, <title> is the start-tag, </title> is the end-tag, title is the element type name, and the element contains the text “Example.”
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How a DTD or schema defines the vocabulary’s rules
A vocabulary’s names do not by themselves specify which combinations are valid. A DTD can declare element types, attributes, and content models. A content model can constrain which child element types an element may contain, including their sequence or alternatives. Attribute-list declarations can specify an element’s attributes, their types, and default values. These rules make a particular vocabulary checkable by software; they do not create a universal catalog of XML names (W3C XML 1.0, section 3.2).
XML Schema is another schema technology used to define constraints on XML documents. The key distinction is that the vocabulary provides the names, while a DTD or schema can express structural expectations for documents that use them.
Example: names versus rules
<book>
<title>Example</title>
</book>Here, book and title are vocabulary names selected for the application. XML syntax specifies how the tags are formed and nested. This snippet alone does not establish a formal vocabulary or prove that title is permitted inside book; a DTD or schema would need to state that rule.
Does every XML document need a DTD?
No. An XML document can be well-formed by following XML’s syntax without being validated against a DTD or schema. Well-formedness concerns whether the markup follows XML syntax; validation checks whether the document also satisfies a declared set of structural rules. A tag set may therefore be described by application conventions even when a document has no attached formal declaration.
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