Most of the content models declared here depend on elements or entities declared in topic.mod.
这里声明的多数内容模型依赖于topic . mod中声明的元素或实体。
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The most common reason for them to be different is if the HTTP Content-type header specifies a different encoding than the one declared in the XML declaration.
This method is particularly useful when processing element-only content (that is, elements declared as EMTPY in the Document Type Declaration, or DTD).