The XML FAQ: Frequently-Asked Questions about the Extensible Markup Language

[basics] What is XML?

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The Extensible Markup Language.

[basics] What is a markup language?

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A way of describing what's what in a document.

[basics] What is XML for (aka ‘Where should I use XML’)?

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XML is for identification, transmission, and storage.

[basics] What is SGML?

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Standard Generalized Markup Language, ISO 8879:1986

[basics] What is HTML?

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HyperText Markup Language, RFC 1866, the language of Web pages.

[basics] Aren't XML, SGML, and HTML all the same thing?

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No, SGML and XML are metalanguages. HTML is an application of them.

[basics] Who is responsible for XML?

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The W3C

[basics] Why is XML such an important development?

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It overcomes the inflexibility of HTML and the complexity of SGML

[basics] Why not just carry on extending HTML?

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HTML is already too overburdened with proprietary add-ons.

[basics] Why should I use XML? (aka ‘What is XML for?)’

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It's a robust, durable, manipulable, and free format for information identification, storage and transfer.

[basics] Where do I find more information about XML?

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At http://xml.coverpages.org/

[basics] Where can I discuss implementation and development of XML?

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On mailing lists, Usenet newsgroups, web-based bulletin-boards, and IRC channels

[basics] What is the difference between XML and C or C++ or Java?

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C and Java are for writing programs; XML is for storing text.

[users] What do I have to do to use XML?

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To read it: an XML browser (eg Firefox or IE). To create: an XML editor (Emacs, Spy, etc).

[users] Should I use XML instead of HTML?

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Yes if you need robustness, accuracy, and persistence.

[users] Someone sent me an XML file. How do I read it?

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Open it in an XML browser or XML editor.

[users] Where can I get an XML browser?

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MSIE 5.5 or 6.*; Mozilla Firefox 0.9.6 up

[users] Do I have to switch from SGML or HTML to XML?

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Not if you don't want to

[users] Can I use XML for ordinary office applications?

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Yes, use Star Office, Open Office, WordPerfect, or even MS-Office (11/XP only).

[authors] Does XML replace HTML?

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No.

[authors] Do I have to know HTML or SGML before I learn XML?

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No, but it's useful.

[authors] What does an XML document actually look like (inside)?

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Pointy brackets like HTML

[authors] How does XML handle white-space in my documents?

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Parsers keep it all. It's up to the application to decide what to do with it.

[authors] Which parts of an XML document are case-sensitive?

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All of it, both markup and text.

[authors] How can I make my existing HTML files work in XML?

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Either make them XHTML or use a different document type.

[authors] Is there an XML version of HTML?

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Yes, XHTML from W3C

[authors] If XML is just a subset of SGML, can I use XML files directly with existing SGML tools?

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Yes, if they are up to date

[authors] I'm used to authoring and serving HTML. Can I learn XML easily?

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Yes

[authors] Can XML use non-Latin characters?

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Yes, this is the default

[authors] What's a Document Type Definition (DTD) and where do I get one?

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A specification of document structure. You can write one or download them.

[authors] Does XML let me make up my own tags?

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Yes but they're not called tags. They're element types.

[authors] How do I create my own document type?

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Analyse the class of documents, and write a DTD or Schema

[authors] How do I write my own DTD?

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Learn XML Declaration Syntax

[authors] Can a root element type be explicitly declared in the DTD?

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No, use the Document Type Declaration.

[authors] I keep hearing about alternatives to DTDs. What's a Schema?

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Like a DTD for validating content as well as structure.

[authors] How do I get XML into or out of my database?

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Ask your database manufacturer

[authors] How will XML affect my document links?

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XML Links are much more powerful, but not yet implemented in browsers

[authors] Can I encode mathematics using XML?

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Yes, using MathML.

[authors] How does XML handle metadata?

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Any way you want.

[authors] Can I use JavaScript, ActiveX, etc in XML files?

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Not in the XML file itself, but via a stylesheet.

[authors] Can I use Java to create or manage XML files?

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Sure.

[authors] How do I execute or run an XML file?

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Not a meaningful question. XML is a data format.

[authors] How do I control formatting and appearance?

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Use a CSS or XSLT stylesheet.

[authors] How do I use graphics in XML?

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Reference them as for HTML or use XLink. Or embed SVG.

[authors] What is parsing and how do I do it in XML?

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Parsing is splitting up information into its component parts

[authors] How do I include one XML file in another?

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Use a general entity, same as for SGML

[authors] When should I use a CDATA Marked Section?

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CDATA is only for text containing markup-like characters.

[authors] How can I handle embedded HTML in my XML?

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Provide for it in the output, use a deep copy, or try disable-output-escaping.

[authors] What are the special characters in XML?

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Just five: < (<), & (&), > (>), " ("), and ' (')

[developers] Where's the spec?

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Right here [http://www.w3.org/TR/REC-xml]

[developers] What are these terms DTDless, valid, and well-formed?

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Well-formed means syntactically correct (DTD or not); valid means a DTD has been used.

[developers] Which should I use in my DTD, attributes or elements?

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See [http://xml.coverpages.org/elementsAndAttrs.html]

[developers] What else has changed between SGML and XML?

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Stricter syntax and no options.

[developers] What's a namespace?

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A named DTD/Schema fragment identified by a URI (URL).

[developers] What XML software is available?

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Thousands of programs: too many to list here.

[developers] What's my information? DATA or TEXT?

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It depends on what you're using it for.

[developers] Do I have to change any of my server software to work with XML?

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Make sure your server sends XML files as text/xml

[developers] Can I still use server-side inclusions?

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Yes, just make sure the output conforms to XML

[developers] Can I (and my authors) still use client-side inclusions?

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Yes, just make sure the output conforms to XML

[developers] I'm trying to understand the XML Spec: why does it have such difficult terminology?

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It has to be formal to be accurate.

[developers] I have to do an overview of XML for my manager/client/investor/advisor. What should I mention?

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Non-proprietary multi-purpose flexible markup

[developers] Is there a conformance test suite for XML processors?

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Yes, see [http://www.oasis-open.org/committees/xmltest/testsuite.htm]

[developers] I've already got SGML DTDs: how do I convert them for use with XML?

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Edit by hand or use software like Near+Far Designer.

[developers] How do I include one DTD (or fragment) in another?

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Use a parameter entity, same as for SGML

[developers] How can I include a conditional statement in my XML?

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You can't: XML isn't a programming language. But you can have conditional criteria in a Schema, DTD, or a processor.

[developers] What's the story on XML and EDI?

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Getting there: still needs more work and agreement.

[appendix] References

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There is a much larger XML and SGML bibliography at [http://xml.coverpages.org/biblio.html].

[appendix] How far are we going?

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To infinity and beyond!

[appendix] Not the XML FAQ

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Infrequently Asked Questions: XLS; XML; Colour; Editing; Games; SOAP; Serving XML; Line breaks; XML Protocol; Javascript; TMX; XUL; XMLHTTP; White-space; Searching; asp.net; Disadvantages; Rendering; Floating-point; Enumeration; XLL; Special characters; Loops; UML; Multimedia; Well-formed; SML; Sorting; WAP; GTT; BPEL; Idempotency; RSS; Variables; Environment variables; Entities; AJaX; Pipelining; Attributes; URI parsing errors; Tables; Byte Order Mark; Patents, Copyright, and Intellectual Property; Escaping; Data export; Data import; Text document formatting functions

[appendix] Lost XML software

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Some of the best software that has disappeared