DTDs describe the structure of a class of documents via element and attribute-list declarations. Element declarations name the allowable set of elements within the document, and specify whether and how declared elements and runs of character data may be contained within each element. Attribute-list declarations name the allowable set of attributes for each declared element, including the
type of each attribute value, if not an explicit set of valid values. DTD markup declarations declare which
element types,
attribute lists,
entities, and
notations are allowed in the structure of the corresponding class of XML documents.
Element type declarations An element type declaration defines an element and its possible content. A valid XML document contains only elements that are defined in the DTD. Various keywords and characters specify an element's content: • EMPTY for specifying that the defined element allows no content, i.e., it cannot have any children elements, not even text elements (if there are whitespaces, they are ignored); • ANY for specifying that the defined element allows any content, without restriction, i.e., that it may have any number (including none) and type of children elements (including text elements); • or an expression, specifying the only elements allowed as direct children in the content of the defined element; this content can be either: • a
mixed content, which means that the content may include at least one text element and zero or more named elements, but their order and number of occurrences cannot be restricted; this can be: • ( #PCDATA ): historically meaning
parsed character data, this means that only one text element is allowed in the content (no quantifier is allowed); • ( #PCDATA |
element name | ... )*: a limited choice (in an exclusive list between parentheses and separated by "|" pipe characters and terminated by the required "*" quantifier) of two or more child elements (including only text elements or the specified named elements) may be used in any order and number of occurrences in the content. • an
element content, which means that there must be no text elements in the children elements of the content (all whitespaces encoded between child elements are then ignored, just like comments). Such element content is specified as
content particle in a variant of
Backus–Naur form without terminal symbols and element names as non-terminal symbols. Element content consists of: • a
content particle can be either the name of an element declared in the DTD, or a
sequence list or
choice list. It may be followed by an optional
quantifier. • a
sequence list means an ordered list (specified between parentheses and separated by a "," comma character) of one or more
content particles: all the
content particles must appear successively as direct children in the content of the defined element, at the specified position and relative order; • a
choice list means a mutually exclusive list (specified between parentheses and separated by a "|" pipe character) of two or more
content particles: only one of these
content particles may appear in the content of the defined element at the same position. • A
quantifier is a single character that immediately follows the specified item it applies to, to restrict the number of successive occurrences of these items at the specified position in the content of the element; it may be either: • + for specifying that there must be one or more occurrences of the item — the effective content of each occurrence may be different; • * for specifying that any number (zero or more) of occurrences is allowed — the item is optional and the effective content of each occurrence may be different; • ? for specifying that there must not be more than one occurrence — the item is optional; • If there is no quantifier, the specified item must occur exactly one time at the specified position in the content of the element. For example: Element type declarations are ignored by
non-validating SGML and XML parsers (in which cases, any elements are accepted in any order, and in any number of occurrences in the parsed document), but these declarations are still checked for form and validity.
Attribute list declarations An attribute list specifies for a given element type the list of all possible attribute associated with that type. For each possible attribute, it contains: • the declared name of the attribute, • its data type (or an enumeration of its possible values), • and its default value. For example: Here are some attribute types supported by both SGML and XML: ; CDATA: this type means
characters data and indicates that the effective value of the attribute can be any textual value, unless the attribute is specified as fixed (the comments in the DTD may further document values that are effectively accepted, but the DTD syntax does not allow such precise specification); ; ID: the effective value of the attribute must be a valid identifier, and it is used to define and anchor to the current element the target of references using this defined identifier (including as document
fragment identifiers that may be specified at end of an URI after a "#" sign); it is an error if distinct elements in the same document are defining the same identifier; the uniqueness constraint also implies that the identifier itself carries no other semantics and that identifiers must be treated as opaque in applications; XML also predefines the standard pseudo-attribute "xml:id" with this type, without needing any declaration in the DTD, so the uniqueness constraint also applies to these defined identifiers when they are specified anywhere in a XML document. ; IDREF or IDREFS: the effective value of the attribute can only be a valid identifier (or a space-separated list of such identifiers) and must be referencing the unique element defined in the document with an attribute declared with the type ID in the DTD (or the unique element defined in an XML document with a pseudo-attribute "xml:id") and whose effective value is the same identifier; ; NMTOKEN or NMTOKENS: the effective value of the attribute can only be a valid name token (or a spaced-separated list of such name tokens), but it is not restricted to a unique identifier within the document; this name may carry supplementary and application-dependent semantics and may require additional naming constraints, but this is out of scope of the DTD; ; ENTITY or ENTITIES: the effective value of the attribute can only be the name of an unparsed external entity (or a space-separated list of such names), which must also be declared in the document type declaration; this type is not supported in HTML parsers, but is valid in SGML and XML 1.0 or 1.1 (including XHTML and
SVG); ; (
value1|...): the effective value of the attribute can only be one of the enumerated list (specified between parentheses and separated by a "|" pipe character) of textual values, where each value in the enumeration is possibly specified between 'single' or "double" quotation marks if it's not a simple name token; ; NOTATION (
notation1|...): the effective value of the attribute can only be any one of the enumerated list (specified between parentheses and separated by a "|" pipe character) of notation names, where each notation name in the enumeration must also be declared in the document type declaration; this type is not supported in HTML parsers, but is valid in SGML and XML 1.0 or 1.1 (including XHTML and SVG). A default value can define whether an attribute must occur (#REQUIRED) or not (#IMPLIED), or whether it has a fixed value (#FIXED), or which value should be used as a default value ("…") in case the given attribute is left out in an XML tag. Attribute list declarations are ignored by
non-validating SGML and XML parsers (in which cases any attribute is accepted within all elements of the parsed document), but these declarations are still checked for well-formedness and validity.
Entity declarations An entity is similar to a
macro. The entity declaration assigns it a value that is retained throughout the document. A common use is to have a name more recognizable than a
numeric character reference for an unfamiliar character. Entities help to improve legibility of an XML text. In general, there are two types: internal and external. •
Internal (parsed) entities are associating a name with any arbitrary textual content defined in their declaration (which may be in the
internal subset or in the
external subset of the DTD declared in the document). When a named entity reference is then encountered in the rest of the document (including in the rest of the DTD), and if this entity name has effectively been defined as a parsed entity, the reference itself is replaced immediately by the textual content defined in the parsed entity, and the parsing continues within this replacement text. •
Predefined named character entities are similar to internal entities: 5 of them however are treated specially in all SGML, HTML and XML parsers. These entities are a bit different from normal parsed entities, because when a named character entity reference is encountered in the document, the reference is also replaced immediately by the character content defined in the entity, but the parsing continues
after the replacement text, which is immediately inserted literally in the currently parsed token (if such character is permitted in the textual value of that token). This allows some characters that are needed for the core syntax of HTML or XML themselves to be escaped from their special syntactic role (notably "&" which is reserved for beginning entity references, "" which delimit the markup tags, and "double" or 'single' quotation marks, which delimit the values of attributes and entity definitions). Predefined character entities also include numeric character references that are handled the same way and can also be used to escape the characters they represent, or to bypass limitations in the character repertoire supported by the document encoding. • In basic profiles for SGML or in HTML documents, the declaration of internal entities is not possible (because external DTD subsets are not retrieved, and internal DTD subsets are not supported in these basic profiles). • Instead, HTML standards predefine a large set of several hundred named character entities, which can still be handled as standard parsed entities defined in the DTD used by the parser. •
External entities refer to external storage objects. They are just declared by a unique name in the document, and defined with a public identifier (an FPI) and/or a system identifier (interpreted as an
URI) specifying where the source of their content. They exist in fact in two variants: •
parsed external entities (most often defined with a SYSTEM identifier indicating the URI of their content) that are
not associated in their definition to a named annotation, in which case validating XML or SGML parsers retrieve their contents and parse them as if they were declared as internal entities (the external entity containing their effective replacement text); •
unparsed external entities that are defined and associated with an annotation name, in which case they are treated as opaque references and signaled as such to the application using the SGML or XML parser: their interpretation, retrieval and parsing is left to the application, according to the types of annotations it supports (see the next section about annotations and for examples of unparsed external entities). • External entities are not supported in basic profiles for SGML or in HTML documents, but are valid in full implementations of SGML and in XML 1.0 or 1.1 (including XHTML and SVG, even if they are not strictly needed in those document types). An example of internal entity declarations (here in an internal DTD subset of an SGML document) is: ]> &question;&signature; Internal entities may be defined in any order, as long as they are not referenced and parsed in the DTD or in the body of the document, in their order of parsing: it is valid to include a reference to a still undefined entity within the content of a parsed entity, but it is invalid to include anywhere else any named entity reference before this entity has been fully defined, including all other internal entities referenced in its defined content (this also prevents circular or recursive definitions of internal entities). This document is parsed as if it was: ]> Why couldn’t I publish my books directly in standard SGML? — William Shakespeare. Reference to the "author" internal entity is not substituted in the replacement text of the "signature" internal entity. Instead, it is replaced only when the "signature" entity reference is parsed within the content of the "sgml" element, but only by validating parsers (non-validating parsers do not substitute entity references occurring within contents of element or within attribute values, in the body of the document. This is possible because the replacement text specified in the internal entity definitions permits a distinction between
parameter entity references (that are introduced by the "%" character and whose replacement applies to the parsed DTD contents) and
general entity references (that are introduced by the "&" character and whose replacement is delayed until they are effectively parsed and validated). The "%" character for introducing parameter entity references in the DTD loses its special role outside the DTD and it becomes a literal character. However, the references to predefined character entities are substituted wherever they occur, without needing a validating parser (they are only introduced by the "&" character).
Notation declarations Notations are used in SGML or XML. They provide a complete reference to unparsed external entities whose interpretation is left to the application (which interprets them directly or retrieves the external entity themselves), by assigning them a simple name, which is usable in the body of the document. For example, notations may be used to reference non-XML data in an XML 1.1 document. For example, to annotate SVG images to associate them with a specific renderer: This declares the
Media Type of external images with this type, and associates it with a notation name "type-image-svg". However, notation names usually follow a naming convention that is specific to the application generating or using the notation: notations are interpreted as additional meta-data whose effective content is an external entity and either a PUBLIC FPI, registered in the catalogs used by XML or SGML parsers, or a SYSTEM URI, whose interpretation is application dependent (here a MIME type, interpreted as a relative URI, but it could be an absolute URI to a specific renderer, or a URN indicating an OS-specific
object identifier such as a UUID). The declared notation name must be unique within all the document type declaration, i.e. in the external subset as well as the internal subset, at least for conformance with XML. Notations can be associated to unparsed external entities included in the body of the SGML or XML document. The PUBLIC or SYSTEM parameter of these external entities specifies the FPI and/or the URI where the unparsed data of the external entity is located, and the additional NDATA parameter of these defined entities specifies the additional notation (i.e., effectively the MIME type here). For example: ]> Within the body of the SGML document, these referenced external entities (whose name is specified between "&" and ";") are
not replaced like usual named entities (defined with a CDATA value), but are left as distinct unparsed tokens that may be used either as the value of an element attribute (like above) or within the element contents, provided that either the DTD allows such external entities in the declared content type of elements or in the declared type of attributes (here the ENTITY type for the data attribute), or the SGML parser is not validating the content. Notations may also be associated directly to elements as additional meta-data, without associating them to another external entity, by giving their names as possible values of some additional attributes (also declared in the DTD within the declaration of the element). For example: ]> &example1SVG; The example above shows a notation named "type-image-svg" that references the standard public FPI and the system identifier (the standard URI) of an SVG 1.1 document, instead of specifying just a system identifier as in the first example (which was a relative URI interpreted locally as a MIME type). This annotation is referenced directly within the unparsed "type" attribute of the "img" element, but its content is not retrieved. It also declares another notation for a vendor-specific application, to annotate the "sgml" root element in the document. In both cases, the declared notation named is used directly in a declared "type" attribute, whose content is specified in the DTD with the "NOTATION" attribute type (this "type" attribute is declared for the "sgml" element, as well as for the "img" element). However, the "title" attribute of the "img" element specifies the internal entity "example1SVGTitle" whose declaration that does not define an annotation, so it is parsed by validating parsers and the entity replacement text is "Title of example1.svg". The content of the "img" element references another external entity "example1SVG" whose declaration also does not define a notation, so it is also parsed by validating parsers and the entity replacement text is located by its defined SYSTEM identifier "example1.svg" (also interpreted as a relative URI). The effective content for the "img" element be the content of this second external resource. The difference with the
GIF image, is that the SVG image is parsed within the SGML document, according to the declarations in the DTD, where the GIF image is just referenced as an opaque external object (which is not parsable with SGML) via its "data" attribute (whose value type is an opaque ENTITY). Only one notation name may be specified in the value of ENTITY attributes (there is no support in SGML, XML 1.0 or XML 1.1 for multiple notation names in the same declared external ENTITY, so separate attributes are needed). However multiple external entities may be referenced (in a space-separated list of names) in attributes declared with type ENTITIES, and where each named external entity is also declared with its own notation). Notations are also completely opaque for XML and SGML parsers, so they are not differentiated by the type of the external entity that they may reference (for these parsers they just have a unique name associated to a public identifier (an FPI) and/or a system identifier (a URI)). Some applications (but not XML or SGML parsers themselves) also allow referencing notations indirectly by naming them in the "URN:
name" value of a standard CDATA attribute, everywhere a URI can be specified. However this behaviour is application-specific, and requires that the application maintains a catalog of known URNs to resolve them into the notations that have been parsed in a standard SGML or XML parser. This use allows notations to be defined only in a DTD stored as an external entity and referenced only as the external subset of documents, and allows these documents to remain compatible with validating XML or SGML parsers that have no direct support for notations. Notations are not used in HTML, or in basic profiles for XHTML and SVG, because: • All external entities used by these standard document types are referenced by simple attributes, declared with the CDATA type in their standard DTD (such as the "href" attribute of an anchor "a" element, or the "src" attribute of an image "img" element, whose values are interpreted as a URI, without needing any catalog of public identifiers, i.e., known FPI) • All external entities for additional meta-data are referenced by either: • Additional attributes (such as
type, which indicates the MIME type of the external entity, or the
charset attribute, which indicates its encoding) • Additional elements (such as
link or
meta in HTML and XHTML) within their own attributes • Standard pseudo-attributes in XML and XHTML (such as
xml:lang, or
xmlns and
xmlns:* for namespace declarations). Even in validating SGML or XML 1.0 or XML 1.1 parsers, the external entities referenced by an FPI and/or URI in declared notations are not retrieved automatically by the parsers themselves. Instead, these parsers just provide to the application the parsed FPI and/or URI associated to the notations found in the parsed SGML or XML document, and with a facility for a dictionary containing all notation names declared in the DTD; these validating parsers also check the uniqueness of notation name declarations, and report a validation error if some notation names are used anywhere in the DTD or in the document body but not declared: • If the application can not use any notation (or if their FPI and/or URI are unknown or not supported in their local catalog), these notations may be either ignored silently by the application or the application could signal an error. • Otherwise, the applications decide themselves how to interpret them, then if the external entities must be retrieved and then parsed separately. • Applications may then signal an error, if such interpretation, retrieval or separate parsing fails. • Unrecognized notations that may cause an application to signal an error should not block interpretation of the validated document using them. == XML DTDs and schema validation ==