What does the standalone directive mean in XML

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What does the  standalone  directive mean in an XML document

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standalone describes if the current XML document depends on an external markup declaration   W3C describes its purpose in  Extensible Markup Language  XML  1 0  Fifth Edition      2 9 Standalone Document Declaration

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The standalone declaration is a way of telling the parser to ignore any markup declarations in the DTD  The DTD is thereafter used for validation only   As an example  consider the humble  lt img gt  tag  If you look at the XHTML 1 0 DTD  you see a markup declaration telling the parser that  lt img gt  tags must be EMPTY and possess src and alt attributes  When a browser is going through an XHTML 1 0 document and finds an  lt img gt  tag  it should notice that the DTD requires src and alt attributes and add them if they are not present  It will also self-close the  lt img gt  tag since it is supposed to be EMPTY  This is what the XML specification means by  markup declarations can affect the content of the document   You can then use the standalone declaration to tell the parser to ignore these rules   Whether or not your parser actually does this is another question  but a standards-compliant validating parser  like a browser  should   Note that if you do not specify a DTD  then the standalone declaration  has no meaning   so there s no reason to use it unless you also specify a DTD

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Markup declarations can affect the   content of the document  as passed   from an XML processor to an   application  examples are attribute   defaults and entity declarations  The   standalone document declaration  which   may appear as a component of the XML   declaration  signals whether or not   there are such declarations which   appear external to the document entity   or in parameter entities   Definition    An external markup declaration is   defined as a markup declaration   occurring in the external subset or in   a parameter entity  external or   internal  the latter being included   because non-validating processors are   not required to read them      http   www w3 org TR xml  sec-rmd

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The intent of the standalone yes declaration is to guarantee that the information inside the document can be faithfully retrieved based only on the internal DTD  i e  the document can  stand alone  with no external references  Validating a standalone document ensures that non-validating processors will have all of the information available to correctly parse the document   The standalone declaration serves no purpose if a document has no external DTD  and the internal DTD has no parameter entity references  as these documents are already implicitly standalone   The following are the actual effects of using standalone yes    Forces processors to throw an error when parsing documents with an external DTD or parameter entity references  if the document contains references to entities not declared in the internal DTD  with the exception of replacement text of parameter entities as non-validating processors are not required to parse this   amp  lt  gt  apos  and quot are the only exceptions When parsing a document not declared as standalone  a non-validating processor is free to stop parsing the internal DTD as soon as it encounters a parameter entity reference  Declaring a document as standalone forces non-validating processors to parse markup declarations in the internal DTD even after they ignore one or more parameter entity references  Forces validating processors to throw an error if any of the following are found in the document  and their respective declarations are in the external DTD or in parameter entity replacement text    attributes with default values  if they do not have their value explicitly provided entity references  other than amp  lt  gt  apos  and quot  attributes with tokenized types  if the value of the attribute would be modified by normalization elements with element content  if any white space occurs in their content    A non-validating processor might consider retrieving the external DTD and expanding all parameter entity references for documents that are not standalone  even though it is under no obligation to do so  i e  setting standalone yes could theoretically improve performance for non-validating processors  spoiler alert  it probably won t make a difference      The other answers here are either incomplete or incorrect  the main misconception is that     The standalone declaration is a way of telling the parser to ignore any markup declarations in the DTD  The DTD is thereafter used for validation only         standalone  yes  means that the XML processor must use the DTD for validation only     Quite the opposite  declaring a document as standalone will actually force a non-validating processor to parse internal declarations it must normally ignore  i e  those after an ignored parameter entity reference   Non-validating processors must still use the info in the internal DTD to provide default attribute values and normalize tokenized attributes  as this is independent of validation

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The standalone directive is an optional attribute on the XML declaration  Valid values are yes and no  where no is the default value  The attribute is only relevant when a DTD is used   The attribute is irrelevant when using a schema instead of a DTD   standalone  yes  means that the XML processor must use the DTD for validation only  In that case it will not be used for    default values for attributes entity declarations normalization  Note that standalone  yes  may add validity constraints if the document uses an external DTD  When the document contains things that would require modification of the XML  such as default values for attributes  and standalone  yes  is used then the document is invalid  standalone  yes  may help to optimize performance of document processing    Source  The standalone pseudo-attribute is only relevant if a DTD is used

[xml] What does the 'standalone' directive mean in XML?

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