DTD - Document Type Definition
A Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes.
A DTD can be declared inline inside an XML document, or as an external reference.
What Does the DOCTYPE Declaration (DTD) Do?
The DOCTYPE Declaration (DTD or Document Type Declaration) does a couple of things:
- When performing HTML validation testing on a web page it tells the HTML (HyperText Markup Language) validator which version of (X)HTML standard the web page coding is supposed to comply with. When you validate your web page the HTML validator checks the coding against the applicable standard then reports which portions of the coding do not pass HTML validation (are not compliant).
- It tells the browser how to render the page in standards compliant mode.
Internal DTD Declaration
If the DTD is declared inside the XML file, it should be wrapped in a DOCTYPE definition with the
following syntax:
<!DOCTYPE root-element [element-declarations]>
Example XML document with an internal DTD:
<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend</body>
</note>
The DTD in the above Tasks is interpreted like as below:
• !DOCTYPE note defines that the root element of this document is note
• !ELEMENT note defines that the note element contains 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"
External DTD Declaration
If the DTD is declared in an external file, it should be wrapped in a DOCTYPE definition with the following
syntax:
<!DOCTYPE root-element SYSTEM "filename">
External DTD Declaration Example
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
And this is the file "note.dtd" which contains the DTD:
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
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