To enable the processor to validate a document from a grammatical point of view, you must provide a series of written statements by using special markers. These can be included directly in the declarative main <! DOCTYPE> or be stored in a separate file with a .Dtd, called Document Type Declaration File. This technique is very convenient when a number of documents has been written with the same language and the same grammar. We have written about language and grammar, it is easy to understand, at this point, developers could be defined as two languages are very similar if not almost identical, especially when referring to a very specific context, however, inventing rules different grammatical. Think, for example, a language to produce labels for audio cassettes or CDs. Eventually the markers would be more or less the same - not all programmers have great imagination - but the grammatical structure may be very different. All markers DTD begin with the characters "<" followed by a name that defines the type of declaration, and ending with the character ">". You can also include a comment in the declaration simply by enclosing it between two pairs of dashes, as follows:
<! ELEMENT ITEM (# PCDATA) - voice of a list of ingredients ->
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