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DTD Introduction
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The main building blocks of both XML and HTML documents are tags like <body>....</body>. The building blocks of XML documentsSeen from a DTD point of view, all XML documents (and HTML documents) are made up by the following simple building blocks:
The following is a brief explanation of each of the building blocks: ElementsElements are the main building blocks of both XML and HTML documents. Examples of HTML elements are "body" and "table". Examples of XML elements could be "note" and "message". Elements can contain text, other elements, or be empty. Examples of empty HTML elements are "hr", "br" and "img". TagsTags are used to markup elements. A starting tag like <element_name> marks up the beginning of an element, and an ending tag like </element_name> marks up the end of an element. Examples:
AttributesAttributes provide extra information about elements. Attributes are always placed inside the starting tag of an element. Attributes always come in name/value pairs. The following "img" element has additional information about a source file:
The name of the element is "img". The name of the attribute is "src". The value of the attribute is "computer.gif". Since the element itself is empty it is closed by a " /". EntitiesEntities are variables used to define common text. Entity references are references to entities. Most of you will know the HTML entity reference: " ". This "no-breaking-space" entity is used in HTML to insert an extra space in a document. Entities are expanded when a document is parsed by an XML parser. The following entities are predefined in XML:
PCDATAPCDATA means parsed character data. Think of character data as the text found between the start tag and the end tag of an XML element. PCDATA is text that will be parsed by a parser. Tags inside the text will be treated as markup and entities will be expanded. CDATACDATA also means character data. CDATA is text that will NOT be parsed by a parser. Tags inside the text will NOT be treated as markup and entities will not be expanded.
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