DIV CSS Standard make Web learning learn the elements of the DTD

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In a DTD, XML elements are declared through a DTD element declaration.

Declaring an Element

In DTDs, XML elements are declared through element declarations. Element declarations use the following syntax:

<! Element element name Category >

Or

<! Element elements name (element content) >

Empty element

Empty elements are declared by category keyword empty:

<! Element name Empty>

Example:

<! ELEMENT BR empty>

XML Example:

<br/>

Only pcdata elements.

Only pcdata elements are declared through the #pcdata in parentheses:

<! Element name (#PCDATA) >

Example:

<! ELEMENT from (#PCDATA) >

Elements with content

The element that is declared by the category keyword any can contain a combination of any resolvable data:

<! Element name Any>

Example:

<! ELEMENT Note any>

Elements with child elements (sequences)

Elements with one or more child elements are defined by the name of the child element in parentheses:

<! element name (child element name) >

Or

<! element name (child element name, child element name,.....) >

Example:

<! ELEMENT Note (to,from,heading,body) >

When child elements are declared in a comma-delimited sequence, the child elements must appear in the document in the same order. In a complete declaration, child elements must also be declared, and child elements can also have child elements. The complete declaration of the "note" element is:

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

To declare an element that appears only once

<! element name (child element name) >

Example:

<! ELEMENT Note >

The example above declares that the message child element must appear once, and must appear once in the "note" element.

Declare elements that appear at least once

<! element name (child element name +) >

Example:

<! ELEMENT Note (message+) >

The plus sign in the example above declares that the message child element must appear at least once within the "note" element.

Declare an element that appears 0 or more times

<! element name (child element name *) >

Example:

<! ELEMENT Note (message*) >

The asterisk in the example above declares that the child element message can appear 0 or more times within the "note" element.

Declare an element that appears 0 times or once

<! element name (child element name?) >

Example:

<! ELEMENT note (message?) >

The question mark in the above example declares that the child element message can appear 0 or one times within the "note" element.

Declare "Non .../..." type of content

Example:

<! ELEMENT Note (To,from,header, (message|body)) >

The above example declares that the "note" element must contain a "to" element, a "from" element, "header" element, and a "body" element that is not a "message" element.

Declaring a mixed type of content

Example:

<! ELEMENT Note (#PCDATA |to|from|header|message) *>

The above example declares that the "note" element can contain pcdata, "to", "from", "header", or "message" that appear 0 or more times.