]> The XML FAQ Frequently-Asked Questions <?site br?>about the Extensible Markup Language Peter Flynn Silmaril Consultants Textual Therapy Division http://silmaril.ie/cgi-bin/blog The following people helped with the original contributions, plus many other members of the W3C XML SIG as well as FAQ readers around the world. Terry Allen Tom Borgman Tim Bray Robin Cover Bob DuCharme Christopher Maden Eve Maler Makoto Murata Peter Murray-Rust Liam Quin Michael Sperberg-McQueen Joel Weber xml sgml html markup structure xslt latex Summary This is the list of Frequently-Asked Questions about the Extensible Markup Language (XML). It has answers to most of the common questions people ask about XML. If you are seeking answers to questions about related areas such as HTML, SGML, CGI scripts, PHP, JSP, Java, databases, or penguins, you may find some pointers, but you should probably look elsewhere as well. The FAQ is intended as a first resource for users, authors, developers, and the interested reader. Details of its organisation, contributors, availability, translations, and revisions are in the Admin sections. Updates to the FAQ are notified to the mailing lists and newsgroups listed in . The full document is available for download in many different formats: see for a list. WTF Seán McGrath suggested: It would be great if FAQs had a WTF section to direct the eyes of the exasperated to Q's with a high desperation index :-), so here are the top dozen most-wanted:
Organisation This FAQ was originally maintained on behalf of the World Wide Web Consortium's XML Special Interest Group. It is divided into four sections: Basics, Users, Authors, and Developers. The questions are numbered independently within each section. As the numbering may change with each version, comments and suggestions should refer to the version number (see ) as well as the section and question number. See for details of citation and reference. Please submit bug reports, suggestions for improvement, and other comments about this FAQ only to the editor. Questions and comments about XML should go to the relevant mailing list or newsgroup. Comments about the XML Specification itself and related specifications should be directed to the W3C. Updates In minor updates the following symbols are used: Additions since the last version are indicated with a plus sign. Changes since the last version are indicated with a plus/minus sign. Deletions retained temporarily for information are indicated with a minus sign. In major updates these are not used because almost every question will have been changed.
Availability This XML document is at . It is XML served converted to HTML by Saxon, so what you read online is HTML in your browser. You can download the unconverted file (avoiding the .xml filetype which over-enthusiastic browsers want to usurp—just rename it after downloading); The DTD is a lightly modified version of DocBook; There is a MindMap version available by clicking on the MindMap logo in the banner at the top of the page. This is an XML format used by FreeMind amd other MindMap software. There are XSL stylesheets for the conversion to HTML and to make the PDF and PostScript versions; A notification of new versions is posted periodically to the Usenet newsgroup, the XML-L, xml-dev, and XSL-List mailing lists, and to the XML/XSL forum on LinkedIn. for printed copies there are versions for A4 PostScript, A4 PDF, Letter PostScript and Letter PDF available. WAP (if anyone's still using it), OEB (eBook), and cHTML versions have been proposed for your handheld devices, and I'm open to offers if anyone wants to write app code. The FAQ is also available in carbon-based toner on flattened dead trees by sending €10 ($15 or equivalent in any convertible currency) to the editor (email first to check rates and postal address).
Translations Those I know about are in: German (partial translation of some questions) [ Karin Driesen ]; Amharic [ Abass Alamnehe ]; Japanese [ Makoto Murata ]; Spanish (currently inaccessible) [ Jaime Sagarduy ]; Korean (currently inaccessible). [ Kangchan Lee ]; Chinese (currently inaccessible) [Neko]. Also in Chinese (also inaccessible) [ Jiang Luqin ]; French [ Jacques André ]; Czech [ Miloslav Nic ]. I would be grateful if the translators of those copies which have become inaccessible would contact me with the new URI.
This document is joint copyright © 1996–2011 by Silmaril Consultants and the editor and is released under the terms of the GNU Free Documentation License (see below). Quotations of the contributions of others remain copyright of the individual contributors. You may copy and distribute this document in any form provided you acknowledge this source and the individual (in the case of a contribution) [see for how] and don't try to pretend you or someone other than the author wrote it. If you want to republish or reprint the FAQ in bulk, or copy all or part of it onto another web site, please ask the editor first to make sure you get the right edition, to make provision for periodic updating, and to ensure you use the correct legal wording. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is available here. You are allowed to distribute, reproduce, and modify it without fee or further requirement for consent subject to the conditions in the section on Modifications. The editor and contributing authors assert their right to be identified as the editor and contributing authors of this document. For citations of this FAQ, use:
Flynn, P (Ed.), The XML FAQ v., Cork, , http://xml.silmaril.ie/, Q.xxx [insert the question title here]
In bibliographic referencing systems this would be something like this (using BIB as an example)
@Booklet{xmlfaq, title = {The XML FAQ}, editor = {Peter Flynn}, howpublished = {Webpage}, address = {Cork}, month = {}, year = , edition = {v}, url = {http://xml.silmaril.ie/}, pages = {Q.#} }
A suitable format for citing individually-authored fragments would be:
AN Other, Title of question. In Flynn, P (Ed.), The XML FAQ v., Silmaril Consultants, Cork,  , Q.xxx. http://xml.silmaril.ie/question.html
In bibliographic referencing systems this would be something like this (again using BIB as an example)
@InCollection{xmlfaq, author = {AN Other}, title = {Title of question}, booktitle = {The XML FAQ}, publisher = {Silmaril Consultants}, month = {}, year = , editor = {Peter Flynn}, volume = {section number}, number = {question number}, address = {Cork}, url = {http://xml.silmaril.ie/section/question/}, edition = {v.} }
0.0 1996-12-27 First test. Unpublished. 0.1 1997-01-31 First draft. Sample questions devised by participants. 0.2 1997-02-03 Revised draft. Additional questions and answers. 0.3 1997-02-17 Extensive revision following comments from the group. Changes to markup and organization. 0.4 1997-02-23 Minor editorial changes 0.5 1997-04-01 Added Multidoc Pro as SGML browser; question on XML math; fixed ambiguity in explanation of NETs; added JUMBO; ERB changes of March 26; more details of linking and tools; adding element declaration minimisation to the forbidden list. 1.0 1997-05-01 Added reference to ToC and printed URIs; added disclaimer at A6; combined old A11 with A5 to explain SGML/XML/HTML; clarified explanation of XML not replacing HTML at C1; added new course and conference at (new) A11; clarified B1, C4, C8; added FPI server at C12; removed examples in C13. 1.1 1997-10-01 No more minimisation parameters in element declarations; parsers must now pass all white-space to the application; everything is now case-sensitive, including all markup; a new proposal for stylesheets: XSL, which combines DSSSL and CSS in an XML format; Java[Script] and and metadata and their use in XML; updated list of software; first XML book is published; new public mailing list XML-L 1.2 1998-02-01 Added a Mac icon (thanks to Martin Winter and others); removed Draft from references to the spec; changed revision colours; the RMD is gone: replaced references to it with standalone; updated some broken URIs; [1.21] minor edits to URIs and updates on translation; added XUA to details of MIME types. 1.3 1998-06-01 Removed the math plugin (Linux Netscape is broken and refused to elide it); updated list of events (need more); fixed some broken URIs; added Spanish and Korean translations and the Annotated Spec; updated details of MS/NS browser development; clarified the use of FPI vs SysiD; updated link to Feb 10 Rec Spec; added pointers to the SGML Decl for XML; updated references to XLink and XPointer; corrected a reference to ancient Sumerian writing; clarified the need for conversion of HTML DTDs to XML. 1.4 1998-10-01 Added maintainer's email address under Availability; Added note about ISO representation and voting on standards; added Greek translation; updated details of conferences; changed the URI for the new SGML/XML Web Pages; updated details of browsers; corrected reference to the SGML omitted features from XML; updated details of converting HTML to XML; added mention of comp.text.xml; extended the questions on graphics and how to use XML with current browsers; added questions on DOM, conformance testing, DTD includes, SGML DTDs into XML, EDI; (1.41) corrected errors in MIME types, URIs, SDD, and images. 1.5 1999-06-01 Added new XML mailing lists in Italian and in French; added details of developer resources in Chinese; two more translations under way (Chinese and Czech); updated links to the question on DTDs; added question on the use of Java to generate and manage XML; added question on when to use attributes and when to use element markup; added question on the use of XML syntax to describe DTD data (schemas); expanded on the explanation of the use of formal language in the spec; added question on the difference between XML and C++; separated information on XML versions of HTML into a separate question. 1.6 2000-07-01 Added French and Czech translations and a Finnish mailing list, and reorganised the list of translations; updated URIs for newsgroups; clarified reference to Unicode; reworded question on terminology; added more links to the question on conformance testing; corrected error in content model example for mixed content; updates to the question on stylesheets; Minor edits to the question on software; major changes to the question on servers and media types; updated question on XML Schemas; added new question on `executing' XML `programs'; replaced the math example with one less likely to distress the gentle susceptibilities of some readers; added a new question on knowing SGML/HTML before XML. 2.0 2001-06-01 DTD changed from DocBook SGML to QAML XML; removed query form due to abuse; most questions revised and in some cases rewritten; updated references to new versions of associated standards, recommendations, and working drafts; added pointer to Jon Noring's Unicode test page and NIST's XSLT/XPath test suite; updated Eve Maler's links to the DTD for the spec; added warnings on speling and punk chew asian; added question on namespaces; fixed bug in question on stylesheets; inserted explanation of `document' vs `data' software; added new mailing list on XSL:FO; updated Robin Cover's URI throughout; updated the question on media types for RFC 3023; Extended question of graphics to cover SVG. For 2.01 there were minor typos, some updated links (to recent versions of the standards, and in the section on More Information), and a few wording changes. Thanks to James Cummings for a very thorough proofread. Editing was done using GNU Emacs and psgml-mode. 2.1 2002-01-01 Added Humanities mailing list; added more references for XML and databases; added the Namespaces FAQ; corrected some misunderstandings in character encodings; changed the editor's email address; added a new question on root elements; updated the XLink to W3C Recommendation; updated the SGML FAQ address; fixed some broken links; added translations into German and Amharic; minor revisions to some wording. Editing this time was done in epcEdit 1.02. V2.11 includes new material on expectations and XML browsers, the removal of a mailing list, and a few corrections to typos and links. Thanks to Seán Cannon and Dave&ers;Nikki for debugging the CSS style-sheet. 3.0 2003-01-01 Added information on Office Applications including Corel, Microsoft, and Sun (to keep alphabetical order :-); updated details of conferences and training; updated browser details; reworded a few ungainly sentences; removed some obsolete URIs (mostly for nice idea sites which died); changed the phrasing of the question on databases; added details on how to do standalone validation to the question on parsing (thanks to Bill Rayer); added question on how to present XML to management (thanks to Tad McClellan); the questions on APIs and the DOM have been subsumed into the question on software, which has been extensively rewritten; added yet more explanation to the section on Unicode; 3.01 fixes minor typos; 3.02 adds updated dates for 2004 events. 3.01 2004-01-01 Minor typographic changes 3.02 2004-01-12 Added updates for 2004 events 4.0 2005-01-01 Went back to DocBook markup using qandaset instead of the QAML that has been used for the last two major releases. Revised text in most sections for clarity in wording, and recast some now-established explanatory material into the past tense. Added new dates for 2005. Added explicit references to the GNU FDL in the legal section. Took the tip on types of XML out into a new question, and added new questions on file inclusions and the use of CDATA Marked Sections. 4.1 2005-05-15 Revised structure and new stylesheet for new location at . The four main sections remain, but the text is served in separate questions and sections rather than one huge file (the PDF remains as a single document, of course). Removed references to the now-defunct Balise language, added a Tip on editor selection and some notes on WYSIWYG XSL[T] editing. 4.2 2005-07-01 Added new RNG mailing list, updated section on Schemas, added links to the XML Declaration for SGML. Retagged personal names for recognition, and ID'd related FAQs. Expanded question on Why XML. Added link to email a page to someone. Added and expanded the tips on ways of getting typeset output, eg . Added new section on special characters. 4.3 2005-09-05 Added the notes culled from failed searches as a Glossary; updated some URLs, and added one for XQuery to the question on databases (thanks, Liam); updated , , , and the question on CDATA Sections. Added a new question on Conditionals. Tightened up on the indexing for searches, including the removal of enclosing quotes, and added a bunch more metadata. 4.31 2005-09-09 Added notes on Pipelining and Attributes. 4.32 2005-09-10 Added details of xml:id to the note on Attributes. 4.33 2005-09-12 Added more keywords, and a tip to the note on asp.net. 4.34 2005-10-01 Split the question on CDATA into two: one for CDATA per se, and one for other ways of handling embedded HTML. Added some more keywords, and revised the questions and . Fixed a minor date bug in the search script. 4.35 2005-10-08 Fixed some broken links and removed a couple of obsolete ones. Added a note about the BOM. 4.36 2005-10-16 Updated dates of events in . 4.37 2005-10-31 Removed ambiguities in . 4.38 2005-11-01 Added personal views on patent, copyright, and intellectual property. 4.39 2005-12-01 Refined some keywords, changed presentations of some examples, reworded a paragraph on treatment of space, and added details of assigning a Schema to an instance. 4.4 2006-01-01 Minor grammatical edits, major changes to the indexing and DC metadata. Added glossary entry on data export to CSV and expanded the description of nodes and the grove. Fixed elusive bug in RSS feed. Added contributor names to search index. 4.41 2006-01-07 Fixed a cross-referencing bug in generated content. 4.5 2006-02-27 Added more keywords taken from failed searches. Expanded on file URIs, the use of compiled DTDs, self-describing data, the boolean nature of parameter entity switches, how to get HTML features (forms, etc). 4.51 2006-02-28 Added explanation of xml:is, xml:space, and xml:lang. Added new question on how to read (open) an XML file you have been sent. 4.52 2006-03-26 Added more keywords and fixed a broken link to the XSL FAQ. 4.53 2006-04-12 Updated details of XML for Safari, and added a curious new enquiry. 4.54 2006-06-01 Corrected an error in the description of xml-stylesheet. Added link targets to Quick Answers. 4.55 2007-08-01 Updated events for 2007–2008. Updated details of ODF and OOXML. Added section on broken software. Revised handling of failed searches. 4.56 2007-08-08 Added details and links for HTML5 4.57 2010-02-27 Updated events, added interim changes to formatting in preparation for extensive relaunch later in 2010. 4.58 2010-04-24 Updated events, removed XML Prague, added Balisage Symposium 5.00 2011-01-09 Removed obsolete information and links, reformatted presentation (thanks to Parker and the PWA for help with the CSS). Moved questions about Java and Javascript from Authors to Developers, and question on running XML to Basics. Renamed IDs appendix to appendices, contrib to contributions, and revhist to revisions so that sectioning can be done by ID rather than number and title. Rewrote search script. Updated events. transformation also updated and the PDFs reset. Basics: general information about XML What is XML? The Extensible Markup Language. XML is the Extensible Markup Language. It improves the functionality of the Web by letting you identify your information in a more accurate, flexible, and adaptable way. It is extensible because it is not a fixed format like HTML (which is a single, predefined markup language). Instead, XML is a metalanguage—a language for describing other languages—which lets you design your own markup languages for limitless different types of documents. XML can do this because it's written in SGML, the international standard metalanguage for text document markup (ISO 8879). What is a markup language? A way of describing what's what in a document. A markup language is a set of words and symbols for describing the identity or function of the component parts of a document (for example this is a paragraph, this is a heading, this is a list, this is the caption of this figure, etc). Programs can use markup with a stylesheet to transform the document into output for screen, print, audio, video, Braille, or reprocessable data formats. Some markup languages (especially those used in wordprocessors) only describe appearances instead (this is italics, this is bold, this has 3mm space below, etc), so these systems can only be used for display, and are not easily re-usable for anything else. XML is sometimes referred to as self-describing because the names of the markup elements can represent the type of content they hold (eg title, chapter, link, etc). What is XML for? XML is for identification, transmission, and storage.
Goal …to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.
Despite early attempts, browsers never allowed other SGML, only HTML (although there were plugins). Browser vendors also allowed (even encouraged) HTML to be corrupt or broken in order to make it easier. This enabled HTML to become widespread, but held development back for over a decade by making it impossible to program for it reliably. XML fixes that by making it compulsory to stick to the rules, and by making the rules much simpler than SGML. But XML is not just for Web pages: in fact it's very rarely used on its own for Web pages because browsers still don't provide reliable support for it. Common uses for XML include: Information identification You can define your own markup, so you can define meaningful names for all your information items. Information storage Because XML is portable and non-proprietary, it can be used to store information across any platforms. Because it is backed by an international standard, it will remain accessible and processable as a data format. Information structure XML structures can nest, so they can be used to store and identify any kind of hierarchical information, especially long, deep, or complex document sets or data sources, which makes it ideal for an information-management back-end to serving the Web. This is one if its most common Web applications, with a transformation system to serve it as HTML until such time as browsers are able to handle XML consistently. Publishing The original goal of XML as defined in the quotation at the start of this section. Combining the three previous topics (identity, storage, and structure) means it is possible to get all the benefits of robust document management and control (with XML) and publish to the Web (as HTML) as well as to paper (as PDF) and to other formats (eg Braille, Audio, etc) from a single source document by using the appropriate stylesheets. Messaging and data transfer XML is also very heavily used for enclosing or encapsulating information in order to pass it between different computing systems which would otherwise be unable to communicate because of their proprietary or secret data formats. By providing a lingua franca for data identity and structure, XML provides a common envelope for inter-process communication (messaging). Web services Building on all of these, as well as its use in browsers, machine-processable data can be exchanged between consenting systems, where before it was only comprehensible by humans (HTML). Weather services, e-commerce sites, blog newsfeeds, AJaX sites, and thousands of other data-exchange services use XML for data management and transmission, and the web browser for display and interaction. What is SGML? Standard Generalized Markup Language, ISO 8879:1986 SGML is the Standard Generalized Markup Language (ISO 8879:1986), the international standard for defining descriptions of the structure of different types of electronic document. There is an SGML FAQ from David Megginson at ; and Robin Cover 's SGML Web pages are at . For a little light relief, try Joe English 's Not the SGML FAQ at . SGML is very large, powerful, and complex. It has been in heavy industrial and commercial use for nearly two decades, and there is a significant body of expertise and software to go with it. XML is a lightweight cut-down version of SGML which keeps enough of its functionality to make it useful but removes all the optional features which made SGML too complex to program for in a Web environment. ISO standards like SGML are governed by the International Organization for Standardization in Geneva, Switzerland, and voted into or out of existence by representatives from every country's national standards body. If you have a query about an international standard, you should contact your national standards body for the name of your country's representative on the relevant ISO committee or working group. If you have a query about your country's representation in Geneva or about the conduct of your national standards body, you should contact the relevant government department in your country, or speak to your public representative. The representation of countries at the ISO is not a matter for this FAQ. Please do not submit queries to the editor about how or why your country's ISO representatives have or have not voted on a specific standard. What is HTML? HyperText Markup Language, RFC 1866, the language of Web pages. HTML is the HyperText Markup Language (RFC 1866), which started as a small application of SGML for the Web, originating with Tim Berners-Lee at CERN in 1989–90. It defines a very simple class of report-style documents, with section headings, paragraphs, lists, tables, and illustrations, with a few informational elements, but very few presentational elements , plus some hypertext and multimedia. See the question on extending HTML. The current recommendation is to use the XML version, XHTML. There is a HTML and XHTML FAQ maintained by Steven Pemberton at Recent moves the W3C have led to the development of a revision of HTML called HTML5. There is an explanation from Elliotte Rusty Harold, and a FAQ from the WhatWG. Aren't XML, SGML, and HTML all the same thing? No, SGML and XML are metalanguages. HTML is an application of them. Not quite; SGML is the mother tongue, and has been used for describing thousands of different document types in many fields of human activity, from transcriptions of ancient Irish manuscripts to the technical documentation for stealth bombers, and from patients' medical and clinical records to musical notation. SGML is very large and complex, however, and probably overkill for most common office desktop applications. XML is an abbreviated version of SGML, to make it easier to use over the Web, easier for you to define your own document types, and easier for programmers to write programs to handle them. It omits all the complex and less-used options of SGML in return for the benefits of being easier to write applications for, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files may still be processed in the same way as any other SGML file (see the question on XML software). HTML is just one of many SGML or XML applications—the one most frequently used on the Web. Technical readers may find it more useful to think of XML as being SGML−− rather than HTML++. (Ed: In respect of this last paragraph, see and .) Who is responsible for XML? The W3C XML is a Recommendation of the World Wide Web Consortium (W3C), and the development of the specification is supervised by an XML Working Group. A Special Interest Group of co-opted contributors and experts from various fields contributed comments and reviews by email. XML is a public format: it is not a proprietary development of any company, although the membership of the WG and the SIG represented companies as well as research and academic institutions. The v1.0 specification was accepted by the W3C as a Recommendation on Feb 10, 1998. Why is XML such an important development? It overcomes the inflexibility of HTML and the complexity of SGML It removes two constraints which were holding back Web developments: dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for; the complexity of full SGML, whose syntax allows many powerful but hard-to-program options. XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for. Why not just carry on extending HTML? HTML is already too overburdened with proprietary add-ons. HTML was already overburdened with dozens of interesting but incompatible inventions from different manufacturers, because it provides only one way of describing your information. XML allows groups of people or organizations to create their own customized markup applications for exchanging information in their domain (music, chemistry, electronics, hill-walking, finance, surfing, petroleum geology, linguistics, cooking, knitting, stellar cartography, history, engineering, rabbit-keeping, mathematics, genealogy, etc). HTML as originally conceived is now well beyond the limit of its usefulness as a way of describing information, and while HTML5 will continue to play an important role for the content it represents, many new applications require a more robust and flexible infrastructure. Why should I use XML? It's a robust, durable, manipulable, and free format for information identification, storage and transfer. Here are a few reasons for using XML (in no particular order). Not all of these will apply to your own requirements, and you may have additional reasons not mentioned here (if so, please let the editor of the FAQ know!). XML can be used to describe and identify information accurately and unambiguously, in a way that computers can be programmed to understand your information (well, at least manipulate as if they could understand it). XML allows documents which are all the same type to be created and handled consistently and without structural errors, because it provides a standardised way of describing, controlling, or allowing/disallowing particular types of document structure. [Note that this has absolutely nothing whatever to do with formatting, appearance, or the actual text or data content of your documents, only the structure of them. If you want styling or formatting, see .] XML provides a robust and durable format for information storage and transmission. Robust because it is based on a proven standard, and can thus be tested and verified; durable (persistent) because it uses plain-text file formats which will outlast proprietary binary ones. XML provides a common syntax for messaging systems for the exchange of information between applications. Previously, each messaging system had its own format and all were different, which made inter-system messaging unnecessarily messy, complex, and expensive. If everyone uses the same syntax it makes writing these systems much faster and more reliable. XML is free. Not just free of charge (free as in beer) but free of legal encumbrances (free as in speech). It doesn't belong to anyone, so it can't be hijacked or pirated. And you don't have to pay a fee to use it (you can of course choose to use commercial software to deal with it, for lots of good reasons, but you don't pay for XML itself). XML information can be manipulated programmatically (under machine control), so XML documents can be pieced together from disparate sources, or taken apart and re-used in different ways. They can be converted into any other format with no loss of information. XML lets you separate form (appearance) from content. Your XML file contains your document information (text, data) and identifies its structure: your formatting and other processing needs are identified separately in a stylesheet or processing system. The two are combined at output time to apply the required formatting to the text or data identified by its structure (location, position, rank, order, or whatever). Any of the Design Goals listed in the XML Specification. Why not just use Word or Notes? Restricted proprietary data formats are unsuitable for durable public information. Information on a network which connects many different types of computer has to be usable on all of them. Public information in particular cannot afford to be restricted to one make or model or manufacturer, or to cede control of its data format to private hands. It is also helpful for such information to be in a form that can be reused in many different ways, as this will minimize wasted time and effort. Proprietary data formats, no matter how well documented or publicized, are simply not an option: their control still resides in private hands and they can be changed or withdrawn arbitrarily without notice. SGML is the international standard for defining this kind of application, and was therefore the natural choice for XML, but those who need an alternative based on different software for other purposes are entirely free to implement similar services using such a system, especially if they are for private use. Where do I find more information about XML? At http://xml.coverpages.org/ Online, there's the XML Specification and the ancillary documentation available from the W3C; Robin Cover's SGML/XML Web pages with an extensive list of online reference material and links to software; and a summary and condensed FAQ from Tim Bray ; and thousands of reference resources available by typing xml into Google or other search engine. For offline resources, see the lists of books, articles, and software for XML in Robin Cover 's SGML and XML Web pages. That site should always be your first port of call. The events listed below are the ones I have been told about. Please mail me if you come across others: there are many other XML events around the world, and most of them are announced on the mailing lists and newsgroups. Events The Balisage conference (the principal technical meeting) will be in Montréal on 1st–5th August 2011. The 2011 annual XML Summer School, organised by Eleven Informatics, will be held in St Edmund Hall, Oxford on 18th–23rd September 2011. The XML-in-Practice 2011 Conference &ers; Exposition (run by IDEAlliance, formerly the GCA) is themed and will be in in October. XML Prague will be held on March 26th &ers; 27th, 2011 at Charles University. Where can I discuss implementation and development of XML? On mailing lists, Usenet newsgroups, web-based bulletin-boards, and IRC channels Two of the principal online support media are Usenet newsgroups and mailing lists. The IRC network is also used to some extent, and most individual projects and programs have their own topic-specific bulletin-boards on their web sites. There is also an unknown number of question-and-answer forum sites which are findable using search engines. For off-line support, see for details of conferences and summerschools. The main Usenet newsgroup is comp.text.xml, although it is less used than formerly. Ask your Internet Provider for access to Usenet, or use a Web interface like the searchable archive maintained by Google. If your browser or mailer doesn't provide newsreading facilities, install one that does, or (better) use a standalone newsreader. The comp.text.sgml is for all practical purposes no longer used. The Microsoft-specific newsgroups are being phased out in favour of web-based forums hosted by Microsoft themselves. The general-purpose mailing list for public discussion is XML-L: to subscribe, visit the Web site and click on the link to join. For those developing software components for XML there is the xml-dev mailing list. You can subscribe by sending a 1–line mail message to saying just SUBSCRIBE. Note that this list is for those people actively involved in developing resources for XML. It is not for general information about XML (use the XML-L list above for that). The XSL-List is for for discussing XSL (both XSLT and XSL:FO). For details of how to subscribe, see . There is a long list of other discussion groups, mailing lists, and forums on Robin Cover's site at . There is a mailing list specifically for XSL-FO only, on eGroups.com. You can subscribe by sending a message to . Be aware that the Yahoo E-Groups XSL-FO list sends out regular automated spam to non-members falsely claiming that they have asked to join. A new Italian mailing list about XML is born: to subscribe, send a mail message without a subject line but with text saying subscribe XML-IT to . Everyone, Italian or not, who wants to debate about XML in our tongue is welcome. Gianni also runs the Humanities XML List. A French mailing list about XML has been created. To subscribe, send subscribe to . Please mention this mailing list to your colleagues who use RELAX NG. Go to: . Mailing lists When you join a mailing list you will be sent details of how to use it. Please Read The Fine Documentation because it contains important information, particularly about what to do if your company or ISP changes your email address. Please note that there is a lot of inaccurate and misleading information published in print and on the Web about subscribing to and unsubscribing from mailing lists. Don't guess: Read The Fine Documentation. What is the difference between XML and C or C++ or Java? C and Java are for writing programs; XML is for storing text. C and C++ (and other languages like FORTRAN, or Pascal, or Visual Basic, or Java or hundreds more) are programming languages with which you specify calculations, actions, and decisions to be carried out in order: XML is a markup specification language with which you can design ways of describing information (text or data), usually for storage, transmission, or processing by a program. It says nothing about what you should do with the data (although your choice of element names may hint at what they are for): Camshaft end bearing retention circlip Ringtown Fasteners Ltd An angle-nosed insertion tool is required for the removal and replacement of this part. ]]> On its own, an SGML or XML file (including HTML) doesn't do anything. It's a data format which just sits there until you run a program which does something with it. See also the question about how to run or execute XML files. (in article ]]>) SGML is a category of document types, with a configurable shared syntax, most of which (like classic HTML) cannot be compiled to produce executable programs. XML is a subcategory of SGML with syntactic restrictions. For example, with XML the vocabulary of a document type is always case sensitive, while with SGML it may be either case sensitive or case insensitive. So, for example, classic HTML is an SGML document type, and XHTML+MathML is an XML document type. While some document types correspond to document markup languages, other document types (like a CTAN catalog entry) are just for structured data[...] I doubt seriously, however, that a computer language like C is in any reasonable sense equivalent to an SGML document type. Does XML replace HTML? No. No. XML itself does not replace HTML. Instead, it provides an alternative which allows you to define your own set of markup elements. HTML is expected to remain in common use on the web, and the current version of HTML (XHTML) is in XML syntax, although HTML5 may depart from this. XML is designed to make the writing of processing software much easier than with SGML, which is what the original HTML was based on. Is there an XML version of HTML? Yes, XHTML from W3C Yes, the W3C Recommendation is XHTML which is a reformulation of HTML 4 in XML 1.0. This specification defines HTML as an XML application, and provides three DTDs corresponding to the ones defined by HTML 4.* (Strict, Transitional, and Frameset). The semantics of the elements and their attributes are as defined in the W3C Recommendation for HTML 4. These semantics were intended to provide the foundation for future extensibility of XHTML. Compatibility with existing HTML browsers is possible by following a small set of guidelines (see the W3C site). Existing users (including everyone who uses a browser) What do I have to do to use XML? To read it: an XML browser (eg Firefox or IE). To create: an XML editor (Emacs, Spy, etc). For the average user of the Web, nothing except use a browser which works with XML (see the question about browsers). Remember some XML components are still being invented or implemented (see the W3C web site), so some features are still either undefined or have yet to be written. You can use XML-conformant browsers to look at some of the stable XML material, such as Jon Bosak's Shakespeare plays and the molecular experiments of the Chemical Markup Language (CML). There are some more example sources listed at , and you will find XML (particularly in the guise of XHTML) being introduced in places where it won't break older browsers. If you want to start preparations for creating your own XML files, see the questions in the Authors' Section and the Developers' Section. What does an XML document actually look like (inside)? Pointy brackets like HTML The basic structure of XML is similar to other applications of SGML, including HTML. The basic components can be seen in the following examples. An XML document starts with an optional Prolog, which can have two (optional) parts: The XML Declaration ]]> which specifies that this is an XML document and that it uses the UTF-8 character repertoire (the default); A Document Type Declaration ]]> which identifies the type of document (here, report) and says where the Document Type Description (DTD) is stored; The Prolog is followed by the Document Instance: A root element, which is the outermost (top level) element (start-tag plus end-tag) which encloses everything else: in the examples below the root elements are conversation and titlepage; A structured mix of descriptive or prescriptive elements enclosing the character data content (text), and optionally any attributes (name="value" pairs) inside some start-tags. XML documents can be very simple, with straightforward nested markup of your own design: Hello, world! Stop the planet, I want to get off! ]]> Or they can be more complicated, with a Schema or DTD, and maybe an internal subset (local DTD changes in [square brackets]); and an arbitrarily complex nested structure: ]> Hello, world! Vitam capias ]]> Or they can be anywhere between: a lot will depend on how you want to define your document type (or whose you use) and what it will be used for. Database-generated or program-generated XML documents used in e-commerce are usually unformatted because they are for machine consumption, not for human reading, and they may use very long names or values, with multiple redundancy and sometimes no character data content at all, just values in attributes: ]]> Should I use XML instead of HTML? Yes if you need robustness, accuracy, and persistence. Yes, if you need robustness, accuracy, and persistence. XML allows authors and providers to design their own document markup instead of being limited by HTML. Document types can be explicitly tailored to an application, so the cumbersome fudging and poodlefaking that has to take place with HTML becomes a thing of the past: your markup can always say what it means. Trivial example: next Monday ]]> Information content can be richer and easier to use, because the descriptive and hypertext linking abilities of XML are much greater than those available in HTML. XML can provide more and better facilities for browser presentation and performance, using XSLT and CSS stylesheets; It removes many of the underlying complexities of SGML-format HTML (which led to them being ignored and broken) in favor of a more flexible model, so writing programs to handle XML is much easier than doing the same for all the old broken HTML. Information becomes more accessible and reusable, because the more flexible markup of XML can be used by any XML software instead of being restricted to specific manufacturers as has become the case with HTML. XML files can be used outside the Web as well, in existing document-handling environments (eg publishing). If your information is transient, or completely static and unreferenced, or very short and simple, and unlikely to need updating, HTML may be all you need. Someone sent me an XML file. How do I read it? Open it in an XML browser or XML editor. If the file is well-formed or valid XML, you can just open it with any XML-conformant browser (see ). This will display the file in an unformatted view, showing all the markup in a format that lets you fold up or unfold the nested hierarchy (click on the little plus and minus symbols), which will at least let you read something. If the file contains a link to an XSLT or CSS stylesheet (and the stylesheet was provided or is web-accessible) then the browser should format the file in a readable manner (but beware that in-browser formatting is not robust). If you want to edit the file, you need an XML editor (see ). Unless you are very skilled with pointy-bracket markup, do not try to edit XML files with non-XML editors. How do I control formatting and appearance? Use a CSS or XSLT stylesheet. In HTML, default styling was built into the browsers because the tagset of HTML was predefined and hardwired into browsers. In XML, where you can define your own tagset, browsers cannot possibly be expected to guess or know in advance what names you are going to use and what they will mean, so you need a stylesheet if you want to display formatted text. Browsers which read XML will accept and use a CSS stylesheet at a minimum, but you can also use the more powerful XSLT stylesheet language to transform your XML into HTML—which browsers, of course, already know how to display (and that HTML can still use a CSS stylesheet). This way you get