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1. A brief introduction to XML file constraints and DTD. We write documents to constrain the writing specifications of an XML document. This is called XML constraints. Commonly used constraint technologies are: XML DTDXML The basic concept of SchemaDTD: document type definition Document type definition DTD files are generally used in conjunction with XML files, mainly to constrain XML files. XML files introduce DTD files, so that XML can customize tags but are constrained by DTD files. For example, in the previous section, XML was used to describe a class's information. If we defined an tag for each student, there would be no syntax error, but it would not conform to the semantics. How could students be described by area? At this time we need to use a DTD file to constrain this XML.
1. XML—Detailed explanation of DTD for XML document constraints
##Introduction: We write documents to constrain the writing specifications of an XML document, which is called XML constraints.
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