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What is a DTD?
A DTD is a Document Type Definition.
A DTD defines the structure and the legal elements and attributes of an XML document.
Why Use a DTD?
With a DTD, independent groups of people can agree on a standard DTD for interchanging data.
An application can use a DTD to verify that XML data is valid.
An Internal DTD Declaration
If the DTD is declared inside the XML file, it must be wrapped inside the definition:
XML document with an internal DTD
<!DOCTYPE note [
]>
Tove
Jani
Reminder
Don’t forget me this weekend
View XML file »
In the XML file, select “view source” to view the DTD.
The DTD above is interpreted like this:
!DOCTYPE note defines that the root element of this document is note
!ELEMENT note defines that the note element must contain four elements: “to,from,heading,body”
!ELEMENT to defines the to element to be of type “#PCDATA”
!ELEMENT from defines the from element to be of type “#PCDATA”
!ELEMENT heading defines the heading element to be of type “#PCDATA”
!ELEMENT body defines the body element to be of type “#PCDATA”
An External DTD Declaration
If the DTD is declared in an external file, the definition must contain a reference to the DTD file:
XML document with a reference to an external DTD
Tove
Jani
Reminder
Don’t forget me this weekend!
View XML file »
And here is the file “note.dtd”, which contains the DTD:
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