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 MIME type of external images with this type, and associates it with a notation name "type-image-svg".
Note that this DOCTYPE can only appear after the optional XML declaration, and before the document body, if the document syntax conforms to XML.
This includes XHTML documents: Finally, the document type definition may include no subset at all; in that case, it just specifies that the document has a single top-level element (this is an implicit requirement for all valid XML and HTML documents, but not for document fragments or for all SGML documents, whose top-level elements may be different from the implied root element), and it indicates the type name of the root element: DTDs describe the structure of a class of documents via element and attribute-list declarations.
A valid XML document contains only elements that are defined in the DTD.
Various keywords and characters specify an element’s content: Note that 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.
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.
An element type declaration defines an element and its possible content.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.Note that 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. 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.