An XML tag set is the collection of element and attribute names used by a particular kind of XML document, together with conventions or declarations that govern how those names are used. XML itself does not provide one universal set of application tags: a document’s vocabulary comes from its application or document type.
What “XML tag set” means
In everyday use, “XML tag set” usually means an XML vocabulary: the names available to a class of documents and the conventions for arranging them. The W3C XHTML 2.0 glossary describes a vocabulary in terms of its element and attribute names and related markup names, while distinguishing that vocabulary from the grammar that prescribes how it is used (W3C XHTML 2.0 glossary).
The names depend on the application. A book catalog might use names such as book and title; another XML application can define an entirely different vocabulary. The names are not universal XML keywords.
XML tags, elements, and names
These terms refer to different parts of XML markup:
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- Tag: the markup in angle brackets. A start-tag begins a non-empty element, and a matching end-tag closes it. An empty-element tag, such as
<item/>, expresses an element with no separate end-tag. - Element type name: the name carried in the tag, such as
item. - Element: the logical structure bounded by tags. Depending on its rules, it can contain text, child elements, or both.
- Attribute: a named property written in an element’s start-tag.
XML defines the syntax for writing these structures, but it generally does not assign application-specific meaning to element and attribute names. The W3C XML 1.0 specification states that it does not constrain their semantics, use, or names beyond syntax, while reserving names beginning with “XML” in any capitalization for standardization (W3C XML 1.0, Fifth Edition, section 2.3).
Does XML have a fixed set of tags?
No. XML defines rules for well-formed markup, not a single global catalog of application tags. The application or document type determines which vocabulary names are meaningful. For example:
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<book>
<title>Example</title>
</book>Here, book and title are names chosen for a particular vocabulary. The example is well-formed markup, but by itself it does not establish that title is allowed inside book or define any validation rules.
How a DTD or schema defines rules
A vocabulary’s names can be accompanied by declarations that make structural and attribute rules explicit. A DTD, for example, can declare element types and their content models, which specify which child element types are allowed and whether they must appear in a particular order or choice. Its attribute-list declarations can define an element’s attributes, their types, and default values (W3C XML 1.0, section 3.2).
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These declarations make a vocabulary’s rules machine-readable; they do not create a universal list of XML names. XML Schema is another schema technology for expressing constraints on XML documents, but not every XML document must use a DTD or schema. A document can be well-formed without being validated against a declared vocabulary.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Syntax, vocabulary, and validation at a glance
| Concept | What it establishes | Example or effect |
|---|---|---|
| XML syntax | How markup is written and structured so it follows XML’s rules. | Matching start- and end-tags, or an empty-element tag such as <item/>. |
| Vocabulary, or informal “tag set” | The element and attribute names used by a particular application or document type, along with usage conventions. | Names such as book and title. |
| DTD or schema declarations | Formal constraints on permitted elements, attributes, and structures. | A rule that specifies which child elements may appear inside book. |
Keep the distinction simple: XML supplies the markup syntax; a vocabulary supplies application-specific names; and declarations or a schema can specify and check structural rules for that vocabulary.
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