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what is html dtd

青灯夜游

Release： 2021-12-03 17:45:49

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In HTML, DTD refers to "Document Type Definition", which is a set of grammatical rules about tags, a verification mechanism for HTML files, and is part of the composition of HTML files. DTD can define legal XML document building blocks, which uses a series of legal elements to define the structure of the document.

what is html dtd

The operating environment of this tutorial: Windows 7 system, HTML5 version, Dell G3 computer.

Document Type Definition (DTD) is a set of syntax rules for tags. It is part of the XML1.0 version specification, is the verification mechanism of html files, and is part of the composition of html files.

DTD can define legal XML document building blocks. It uses a series of legal elements to define the structure of the document.

DTD can be declared in an XML document as a line or as an external reference.

Internal DOCTYPE declaration

If the DTD is included in your XML source file, it should be wrapped in a DOCTYPE declaration using the following syntax:

<!DOCTYPE root-element [element-declarations]>

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Example of XML document with DTD (please open in IE5 and higher, and choose to view source code):

<?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&#39;t forget me this weekend</body>
</note>

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Open this XML file in your browser, and choose " View source code" command.

The above DTD is explained as follows:

!DOCTYPE note (second line) defines this document as a note type document.
!ELEMENT note (third line) defines the note element as having four elements: "to, from, heading,, body"
!ELEMENT to (the fourth line) defines the to element as "#PCDATA" type
!ELEMENT from (the fifth line) defines the from element as "#PCDATA" type
!ELEMENT heading (line 6) defines the heading element as "#PCDATA" type
!ELEMENT body (line 7) defines the body element as "# PCDATA" type

External Document Declaration

If the DTD is located outside the XML source file, then it should be encapsulated in a DOCTYPE definition:

<!DOCTYPE root-element SYSTEM "filename">

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This XML document is the same as the above XML document, but has an external DTD: (Click to open the file and select the "View Source Code" command.)

<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don&#39;t forget me this weekend!</body>
</note>

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This is the "note.dtd" file containing the DTD:

<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>

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Why use DTD?

With a DTD, each of your XML files can carry a description of its own format.
With DTD, independent groups can consistently use a standard DTD to exchange data.
Your application can also use a standard DTD to verify data received from the outside.
You can also use DTDs to validate your own data.

DTD has three document types: S (Strict), T (Transitional), and F (Frameset).

#Strict: Use this type if you need clean markup without clutter in the presentation layer. Please use with Cascading Style Sheets (CSS)
Transitional: DTD can contain rendering attributes and elements that the W3C expects to be moved into the style sheet. If your readers are using browsers that do not support Cascading Style Sheets (CSS) and you have to use HTML's rendering features use
#Frameset: DTD should be used with Framework documentation. Except that the frameset element replaces the body element, the Frameset DTD is equivalent to the Transitional DTD

html5 basically does not have the strict requirements of XHTML 1.0 Transitional, and has simplified many things and can be used directly <!DOCTYPE HTML>

Recommended tutorial: "html video tutorial"

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