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The XML FAQ

Version 1.41 (6 October 1998)

Maintained on behalf of the World Wide Web Consortium's XML Special Interest Group by Peter Flynn, (University College Cork), with the collaboration of Terry Allen, (), Tom Borgman, (Harlequin Ltd), Tim Bray, (Textuality, Inc), Robin Cover, (Isogen, Inc), Christopher Maden, (O'Reilly & Associates), Eve Maler, (Arbortext, Inc), Peter Murray-Rust, (Nottingham University), Liam Quin, (Groveware), Michael Sperberg-McQueen, (University of Illinois at Chicago), Joel Weber, (MIT), Murata, Makoto (Fuji Xerox Information Systems), and many other members of the XML Special Interest Group of the W3C as well as FAQ readers around the world. Please use the form at the end for any corrections or additions.

Recent changes

1 October 1998

• Updated details of conferences
• Changed the URL 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
• 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, URLs, SDD, and images
• Typos and minor corrections

Paragraphs which have been added since the last version are shown prefixed with a pilcrow (¶). Paragraphs which have been changed since the last version are shown prefixed with a section sign (§). Paragraphs marked for future deletion but retained at the moment for information are prefixed with a plus/minus sign (±).

Summary

This document contains the most frequently-asked questions (with answers) about XML, the Extensible Markup Language. It is intended as a first resource for users, developers, and the interested reader, and should not be regarded as a part of the XML Specification.

Organization

The FAQ is divided into four parts: a) General, b) User, c) Author, and d) Developer. The questions are numbered independently within each section. As the numbering may therefore change with each version, comments and suggestions should refer to the version number (see Revision History above) as well as the Part and Question Number.

§ There is a form at the end of this document which you can use to submit bug reports, suggestions for improvement, and other comments relating to this FAQ only (or you can mail the maintainer direct at pflynn@imbolc.ucc.ie). Comments about the XML Specification itself should be sent to the W3C.

Availability

The SGML file for use with any conforming SGML system is available at http://www.ucc.ie/xml/faq.sgml (this can also be used online with SGML browsers like Panorama or Multidoc Pro; you can also download the DTD and stylesheet installation self-extractor for faster local access with these browsers, or the DTD set as ASCII files).

The same text is available in an HTML version for use with any HTML browser (eg Netscape Navigator, Microsoft Internet Explorer, Spry Mosaic, NCSA Mosaic, Lynx, Opera, GNUscape Navigator etc) at http://www.ucc.ie/xml/.

An XML version will be produced when DTDs and browsers are more widely available to handle it.

A plaintext (ASCII) version is available from the Web and (eventually) by anonymous FTP to one of several FAQ repositories. The versions above are also available by electronic mail to the WebMail server (for users with email-only access). The plaintext version is posted to comp.text.xml monthly for the archives.

For printed copies there are PostScriptTM versions for A4 and Letter sizes of paper.

The document is also available in oil-based toner on flattened dead trees by sending \$10 (or equivalent) to the editor (email first to check currency and postal address).

§ Thanks to Murata Makoto for making this document available in Japanese (see http://www.fxis.co.jp/DMS/sgml/xml/xmlfaq.html); to Jaime Sagarduy of the Universidad de Deusto, Bilbao for the translation into Spanish (see http://slug.ctv.es/~olea/sgml-esp/xfaq13.html); to the Techno 2000 Project for the Korean version (see http://xml.t2000.co.kr/faq/index.html); and to Tim Bray for the Annotated Spec at http://www.xml.com/axml/testaxml.htm. A Greek version is in preparation.

You can download the XML logo and an icon for your files in ICO (Microsoft Windows), Mac, or XBM (X Window system) format.

A. General questions

A.1   What is XML?

XML is the Extensible Markup Language' (extensible because it is not a fixed format like HTML). It is designed to enable the use of SGML on the World Wide Web.

It's actually slightly misnamed: XML itself is not a single markup language: it's a metalanguage to let you design your own markup language. A regular markup language defines a way to describe information in a certain class of documents (eg HTML). XML lets you define your own customized markup languages for many classes of document. It can do this because it's written in SGML, the international standard metalanguage for markup languages.

A.2   What is XML for?

XML is designed to make it easy and straightforward to use SGML on the Web: easy to define document types, easy to author and manage SGML-defined documents, and easy to transmit and share them across the Web.'

It defines an extremely simple dialect of SGML which is completely described in the XML Specification. The goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML.'

For this reason, XML has been designed for ease of implementation, and for interoperability with both SGML and HTML' [quotes from the XML spec].

A.3   What is SGML?

SGML is the Standard Generalized Markup Language (ISO 8879), the international standard for defining descriptions of the structure and content of different types of electronic document. There is an SGML FAQ at http://www.infosys.utas.edu.au/info/sgmlfaq.txt which is posted every month to the comp.text.sgml newsgroup, and the SGML Web pages are at http://www.oasis-open.org/cover/sgml-xml.html.

ISO standards 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 contry, or speak to your public representative or other politician.

The representation of countries at the ISO is not a matter for this FAQ. Please do not submit queries to the maintainer about how or why your ISO representatives have or have not voted.

A.4   What is HTML?

HTML is the HyperText Markup Language (RFC 1866), a specific application of SGML used in the World Wide Web.

A.5   Aren't XML, SGML, and HTML all the same thing?

Not quite. SGML is the mother tongue', used for describing thousands of different document types in many fields of human activity, from transcriptions of ancient Sumerian tablets to the technical documentation for stealth bombers, and from patients' clinical records to musical notation.

HTML is just one of these document types, the one most frequently used in the Web. It defines a single, fixed type of document with markup that lets you describe a common class of simple office-style report, with headings, paragraphs, lists, illustrations, etc, and some provision for hypertext and multimedia.

XML is an abbreviated version of SGML, to make it easier for you to define your own document types, and to make it easier for programmers to write programs to handle them. It omits the more complex and less-used parts of SGML in return for the benefits of being easier to write applications, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files may still be parsed and validated the same as any other SGML file (see the question on XML software).

Programmers may find it useful to think of XML as being SGML-- rather than HTML++.

A.6   Who is responsible for XML?

§ XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is being supervised by their 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. The v1.0 specification was accepted by the W3C as Recommendation on Feb 10, 1998.

A.7   Why is XML such an important development?

It removes two constraints which are holding back Web developments:

1. dependence on a single, inflexible document type (HTML);

2. the complexity of full SGML, whose syntax allows many powerful but hard-to-program options.

XML simplifies the levels of optionality in SGML, and allows the development of user-defined document types on the Web.

A.8   How does XML make SGML simpler and still let you define your own document types?

§ To make SGML simpler, XML redefines some of SGML's internal values and parameters, and removes a large number of the more complex and sometimes less-used features which made it harder to write processing programs (see http://www.w3.org/TR/NOTE-sgml-xml-971215. ).

§ Although it retains all of SGML's structural abilities which let you define your own document type, it also introduces a new class of document which does not require you to use a predefined document type. See the questions about valid' and well-formed' documents, and how to define your own document types in the Developers' Section.

A.9   Why not just carry on extending HTML?

HTML is already overburdened with dozens of interesting but often incompatible inventions from different manufacturers, because it provides only one way of describing your information.

XML will allow groups of people or organizations to create their own customized markup languages 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, etc).

HTML is at the limit of its usefulness as a way of describing information, and while it will continue to play an important role for the content it currently represents, many new applications require a more robust and flexible infrastructure.

A.10   Why do we need all this SGML stuff? Why not just use Word or Notes?

Information on a network which connects many different types of computer has to be usable on all of them. Public information 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 can minimize wasted time and effort.

SGML is the international standard which is used for defining this kind of application, but those who need an alternative based on different software are entirely free to implement similar services using such a system, especially if they are for private use.

§ Online, there's the XML Specification and ancillary documentation available from the W3C; an XML section with an extensive list of online reference material in Robin Cover's SGML pages; and a summary and condensed FAQ from Tim Bray.

§ The items listed below are the ones I have been told about: please mail me if you come across others.

• § The annual SGML Conference run by the Graphic Communications Association has been renamed the XML Conference. XML'98 is being held in Chicago on November 16–19 and consists of two conferences in one: the XML Conference '98 (Monday to Wednesday) and Markup Technologies '98 (Thursday and Friday).

• § SGML/XML Asia/Pacific is in Sydney on October 12–14.

• § XML Europe '99 is in Granada, April 26–30 (not Rome as previously mentioned).

Further details of these on the GCA's Web site.

§ There is a list of books and articles on XML in Robin Cover's SGML and XML pages: this site should always be your first port of call.

A.12   Where can I discuss implementation and development of XML?

There is a mailing list called xml-dev for those committed to developing components for XML. You can subscribe by sending a 1-line mail message to majordomo@ic.ac.uk saying:

subscribe xml-dev yourname@yoursite

The list is hypermailed for online reference at http://www.lists.ic.ac.uk/hypermail/xml-dev/.

§ Note that this list is for those people actively involved in developing resources for XML. It is not for general information about XML (see this FAQ and other sources) or for general discussion about SGML implementation and resources (see below).

§ There is a general-purpose mailing list XML-L for public discussions: to subscribe, send a 1-line mail message to LISTSERV@listserv.heanet.ie saying

subscribe XML-L forename surname

(substituting your own forename and surname). To unsubscribe, send a 1-line message to the same address saying

unsubscribe XML-L

The Usenet newsgroup comp.text.sgml has been supplemented by comp.text.xml (if this is not yet available on your local news server, ask your Internet Provider to add it).

B. Users of SGML (including browsers of HTML)

B.1   What do I have to do to use XML?

§ There's not much you can do yet unless you want to experiment, as XML is still being implemented. However, there are already some pilot browsers, so you can experiment with them. As developers make progress, more software should start to appear, and you may be able to download browsers and use them to browse the Web much as you do with current applications.

§ You can use the pilot browsers to look at some of the emerging 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 http://www.oasis-open.org/cover/xml.html#examples.

If you want to start preparations for writing your own XML, see the questions in the Authors' Section.

B.2   Why should I use XML instead of HTML?

• Authors and providers can design their own document types using XML, instead of being stuck with HTML. Document types can be explicitly tailored to an audience, so the cumbersome fudging that has to take place with HTML to achieve special effects should become a thing of the past: authors and designers will be free to invent their own markup elements;

• Information content can be richer and easier to use, because the hypertext linking abilities of XML are much greater than those of HTML.

• XML can provide more and better facilities for browser presentation and performance;

• It removes many of the underlying complexities of SGML in favor of a more flexible model, so writing programs to handle XML will be much easier than doing the same for full SGML.

• Information will be 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.

• § Valid XML files are kosher SGML, so they can be used outside the Web as well, in an SGML environment.

B.3   Where can I get an XML browser?

§ Remember the XML specification is still new, so a lot of what you see now is experiemental. As with HTML, there won't be just one browser, but many. However, because the potential number of different XML applications is not limited, no single browser can be expected to handle 100% of everything.

§ The generic parts of XML (eg parsing, tree management, searching, formatting, etc) are being combined into general-purpose browser libraries or toolkits to make it easier for developers to take a consistent line when writing XML applications. Such applications can then be customized by adding semantics for specific markets, or using languages like Java to develop plugins for generic browsers and have the specialist modules delivered transparently over the Web.

§ MSIE4 and 5 handle some Microsoft-oriented XML but currently render to HTML for display. There are some details of development work at Microsoft and of how to enable it. Arbortext's XML Stylesheet program also has details of how to use it with XSL. You will also need the ActiveX controls and XSL codebase. Microsoft are also the architects of a suggestion that you can embed fragments of XML in HTML files because current HTML-only browsers simply ignore element markup which they don't recognize.

§ The publicly-released Netscape code (Mozilla) has resulted in a test XML implementation including an application of RDF plus James Clark's expat XML parser.

§ See also the notes on software for authors and developers, and the more detailed list on the XML pages in the SGML Web site at http://www.oasis-open.org/cover/xml.html.

B.4   Do I have to switch from SGML or HTML to XML?

§ No, existing SGML and HTML applications software will continue to work with existing files. But as with any enhanced facility, if you want to view or download and use XML files, you will need to add XML-aware software as it becomes available.

C. Authors of SGML (including writers of HTML)

Authors should also read the Developers' Section, which contains further information about the internals of XML files.

C.1   Does XML replace HTML?

§ 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 for some time to come, and DTDs for HTML will be available in XML versions as well as in the original SGML versions. XML is designed to make the writing of DTDs much simpler than with full SGML.

§ Work is going on to produce XML versions of HTML and other popular DTDs, but this may not take off until more stable software is available. Watch comp.text.sgml, comp.text.xml, XML-L, and xml-dev for announcements.

C.2   What does an XML document look like inside?

The basic structure is very similar to most other applications of SGML, including HTML. XML documents can be very simple, with no document type declaration, and straightforward nested markup of your own design:

<?xml version="1.0" standalone="yes"?>
<conversation>
<greeting>Hello, world!</greeting>
<response>Stop the planet, I want to get off!</response>
</conversation>

Or they can be more complicated, with a DTD specified, and maybe an internal subset, and a more complex structure:

<?xml version="1.0" standalone="no" encoding="UTF-8"?>
<!DOCTYPE titlepage SYSTEM "http://www.frisket.org/dtds/typo.dtd"
<titlepage>
<white-space type="vertical" amount="36"/>
alignment="centered">Hello, world!</title>
<white-space type="vertical" amount="12"/>
<!-- In some copies the following decoration is
hand-colored, presumably by the author -->
<image location="http://www.foo.bar/fleuron.eps" type="URL" alignment="centered"/>
<white-space type="vertical" amount="24"/>
</titlepage>

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. See the question on valid and well-formed files.

David Brownell adds: XML that's just' well-formed doesn't need to use a Standalone Document Declaration at all. Such declarations are there to permit certain speedups when processing documents while ignoring external parameter entities[mdash ]basically, you can't rely on external declarations in standalone documents. The types that are relevant are entities (as noted) and attributes. Standalone documents must not require any kind of attribute value normalization or defaulting, otherwise they are invalid.

C.3   How does XML handle white-space in my documents?

The SGML rules regarding white-space have been changed for XML, so all white-space, including linebreaks, TAB characters, and regular spaces, is passed by the parser unchanged to the application (browser, formatter, viewer, etc). This means:

• insignificant' white-space between structural elements (those which can contain only other elements, not text data, sometimes called element content') will get passed to the application (under full' SGML this white-space is suppressed);

• significant' white-space within elements which can contain text and markup mixed together (mixed content' or PCDATA [parsed character data]) will still get passed to the application as before.

<chapter>
<section>
<title>
My title for Section
1.
</title>
<para>
...
</para>
</section>
</chapter>

The parser must, however, still inform the application what white-space occurred in element content, if known. (Users of full' SGML may recognize that this information was not in the ESIS, but it is in the grove.) In the above example, the application will receive all the pretty-printing linebreaks, TABs, and spaces between the elements as well as those embedded in the section title. It is the function of the application (browser, formatter, viewer, etc) to decide which type of white-space to discard and which to retain.

C.4   Which parts of an XML document are case-sensitive?

All of an XML file is case-sensitive, both markup and text. This is significantly different from HTML and many other SGML document types. It was introduced to allow markup in non-Latin-alphabet scripts and to obviate problems with case-folding in scripts which are caseless.

• § Element type names (used in start-tags and end-tags) are case-sensitive: you must stick with whatever combination of upper- or lower-case you use to define them (either by usage or in a DTD);

• § For well-formed files with no DTD, the first occurrence of an element type name defines the casing. So you can't say <BODY> . . . </body>: upper- and lower-case must match; thus <IMG/> and <img/> are two different element types;

• Attribute names are also case-sensitive, on a per-element basis: for example <PIC width="7in"/> and <PIC WIDTH="6in"/> in the same file exhibit two separate attributes, because the different casings of width and WIDTH distinguish them;

• § Attribute values are also case-sensitive. Character data values (eg HRef="MyFile.SGML") are exactly as before, but ID and IDREF attributes are now case-sensitive and no longer get folded to uppercase for comparisons;

• All entity names (&Aacute;), and your data content (your text), are case-sensitive, exactly as before.

C.5   How can I make my existing HTML files work in XML?

§ Make them well-formed (see below). A DTD is optional in XML, but HTML files converted to XML format currently have to be DTDless anyway. There are no XML versions of the current SGML-based HTML DTDs yet (on their way) becausee they need to be substantially edited to remove their dependence on those features of SGML which are excluded from XML.

It is necessary to convert existing HTML files to be well-formed because XML does not allow end-tag minimization (missing </p>, etc) which is allowed in most HTML DTDs. Many HTML authoring tools already produce almost (but not quite) well-formed XML.

§ If you want to move your files out of HTML into some other DTD, there is a pilot site run by CommerceNet (http://www.xmlx.com/) for the exchange of XML DTDs, and a pilot FPI server at http://www.ucc.ie/cgi-bin/public with several common SGML DTDs to start from.

If you have created your HTML files conforming to one of the several HTML Document Type Definitions (DTDs), and they validate OK, then they can be converted as follows:

• replace the DOCTYPE declaration and any internal subset (basically everything within the first set of angled brackets <!DOCTYPE HTML...>) with the XML Declaration <?xml version="1.0" standalone="yes"?>

• change any EMPTY elements (eg <ISINDEX>, <BASE>, <META>, <LINK>, <NEXTID> and <RANGE> in the header, and <IMG>, <BR>, <HR>, <FRAME>, <WBR>, <BASEFONT>, <SPACER>, <AUDIOSCOPE>, <AREA>, <PARAM>, <KEYGEN>, <COL>, <LIMITTEXT>, <SPOT>, <TAB>, <OVER>, <RIGHT>, <LEFT>, <CHOOSE>, <ATOP>, and <OF> in the body) so that they end with />', for example <IMG SRC="mypic.gif" alt="Picture"/>

• § ensure there are correctly-matched explicit end-tags for all non-empty elements; eg every <P> must have a </P>, etc: this can be automated by a normalizer program like sgmlnorm (part of SP) or the sgml-normalize function in an editor like Emacs/psgml;

• § escape all < and & markup characters as &lt; and &amp;

• ensure all attribute values are in quotes;

• ensure all occurrences of all element names in start-tags and end-tags match with respect to upper- and lower-case and that they are consistent throughout the file;

• ensure all attribute names are similarly in a consistent case throughout the file.

Be aware that many HTML browsers may not accept XML-style EMPTY elements with the trailing slash, so the above changes are not backwards-compatible. An alternative is to add a dummy end-tag to all EMPTY elements, so <IMG src="foo.gif"> becomes <IMG src="foo.gif"></IMG>.

If you have a lot of valid HTML files, you could write a script in an SGML conversion system to do this (such as Omnimark, Balise, SGMLC, or a system using one of the SGML Perl libraries), or you could even use editor macros if you know what you're doing.

§ If your HTML files are invalid (HTML created by most WYSIWYG editors is invalid) then they will almost certainly have to be converted manually, although if the deformities are regular and carefully constructed, the files may actually be almost well-formed, and you could write a program or script to do as described above. To test for invalidity and non-conformance, check the following:

• § do the files contain markup syntax errors? For example, are there any backslashes instead of forward slashes on end-tags; or elements which nest incorrectly (eg <B>an element which starts <I>inside one element</B> but ends outside it</I>)?

• § do the files contain markup which conflicts with the HTML DTDs, such as headings inside paragraphs, or list items outside list environments?

• do the files use elements which are not in any DTD? Although this is easy to transform to a DTDless well-formed file (because you don't have to define elements in advance) most proprietary [browser-specific] extensions have never been formally defined, so it is often impossible to work out where they can meaningfully be used.

Markup which is valid but which is meaningless or void may need to be edited out before conversion (such as repeated empty paragraphs or linebreaks, empty tables, invisible spacing' GIFs etc: XML uses stylesheets, so you won't need any of these)

See the rules for well-formed' XML files for details of what you need to check in XML when converting.

Note there are XML versions of the HTML DTD in preparation:

• § Ben Trafford is developing an XML version of HTML 3.2

C.6   If XML is just a subset of SGML, can I use XML files directly with SGML tools?

§ Yes, provided: a) the document has a valid Document Type Definition (DTD), ie the files are valid, not just well-formed; and b) you use software which knows about the new WebSGML Adaptations to ISO 8879 (features needed to support XML, such as the special form for EMPTY elements; some aspects of the SGML Declaration such as NAMECASE GENERAL NO; multiple attribute declarations, etc).

At the moment there are few tools which handle XML files unchanged because of the format of these EMPTY elements, but this is changing. The nsgmls parser has an experimental XML conformance switch, and the first XML-specific editors and parsers are appearing (see the question on software).

±

C.7   I'm used to authoring and serving HTML. Can I learn XML easily?

Yes, very easily, but at the moment there is still a need for tutorials, simple tools, and more examples of XML documents. Well-formed XML documents may look similar to HTML except for some small but very important points of syntax.

§ As every user community can define their own document types, it should be much easier to learn, because element names can be picked for relevance.

C.8   Will XML be able to use non-Latin characters?

§ Yes, the XML Specification explicitly says XML uses ISO 10646, the international standard 31-bit character repertoire which covers most human (and some non-human) languages. This is currently congruent with Unicode.

The spec says (2.2): All XML processors must accept the UTF-8 and UTF-16 encodings of ISO 10646 . . . '. UTF-8 is an encoding of Unicode into 8-bit characters: the first 128 are the same as ASCII, the rest are used to encode the rest of Unicode into sequences of between 2 and 6 bytes. UTF-8 in its single-octet form is therefore the same as ISO 646 IRV (ASCII), so you can continue to use ASCII for English or other unaccented languages using the Latin alphabet. Note that UTF-8 is incompatible with ISO 8859-1 (ISO Latin-1) after code point 126 decimal (the end of ASCII). UTF-16 is like UTF-8 but with a scheme to represent the next 16 planes of 64k characters as two 16-bit characters.

 . . . the mechanisms for signalling which of the two are in use, and for bringing other encodings into play, are [ . . . ] in the discussion of character encodings.' The XML Specification explains how to specify in your XML file which coded character set you are using.

§ Use of UCS-4 can only legally be specified in SGML or XML when the WebSGML Adaptations to ISO 8879 are implemented: this enables numbers longer than eight digits to be used in the SGML Declaration.

Regardless of the specific encoding used, any character in the ISO 10646 character set may be referred to by the decimal or hexadecimal equivalent of its bit string': so no matter which character set you personally use, you can still refer to specific individual characters from elsewhere in the encoded repertoire by using &#dddd; (decimal character code) or &#xHHHH; (hexadecimal character code, in uppercase).

The terminology can get confusing, as can the numbers: see the ISO 10646 Concept Dictionary. Rick Jelliffe has XML-ized' the ISO character entity sets.

C.9   What's a Document Type Definition (DTD) and where do I get one?

§ A DTD is usually a file (or several files to be used together) which contains a formal definition of a particular type of document. This sets out what names can be used for element types, where they may occur, and how they all fit together. For example, if you want a document type to describe <LIST>s which contain <ITEM>s, part of your DTD would contain something like

<!ELEMENT list (item)+>
<!ELEMENT item (#pcdata)>

This defines lists containing items and items containing text. XML is a formal language which lets processors automatically parse a DTD and then use that information to identify where every element type comes and how each relates to the other, so that stylesheets, navigators, browsers, search engines, databases, printing routines, and other applications can be used.

In effect, a DTD provides applications with advance notice of what names and structures can be used in a particular document type. Using a DTD means you can be certain that all documents which belong to a particular type will be constructed and named in a conformant manner.

± [Note that in XML, there are no minimization parameters (-' and O' characters in element definitions between element name and content model), because all elements except empty ones must have both start-tag and end-tag present at all times.]

§ There are thousands of SGML DTDs already in existence in all kinds of areas (see the SGML Web pages for examples). Many of them can be downloaded and used freely; or you can write your own. As with any language, you need to learn it to do this: but XML is much simpler than full SGML: see the list of restrictions which shows what has been cut out. Existing SGML DTDs need to be converted to XML for use with XML systems: read the question on converting SGML DTDs to XML, and expect to see announcements of popular DTDs becoming available in XML format.

C.10   How will XML affect my document links?

§ The linking abilities of XML systems are much more powerful than those of HTML, so you'll be able to do much more with them. Existing HREF-style links will remain usable, but new linking technology is based on the lessons learned in the development of other standards involving hypertext, such as TEI and HyTime, which let you manage bidirectional and multi-way links, as well as links to a span of text (within your own or other documents) rather than to a single point. This is already implemented for SGML in browsers like DynaText, Panorama and Multidoc Pro.

§ The XML Linking Specification (XLink) and XML Extended Pointer Specification (XPointer) documents contain a detailed draft specification. An XML link can be either a URL or a TEI-style Extended Pointer (XPointer), or both. A URL on its own is assumed to be a resource (as with HTML); if an XPointer follows it, it is assumed to be a sub-resource of that URL; an XPointer on its own is assumed to apply to the current document.

An XPointer is always preceded by one of #, ?, or |. The # and ? mean the same as in HTML applications; the | means the sub-resource can be found by applying the XPointer to the resource, but the method of doing this is left to the application.

§ The TEI Extended Pointer Notation (EPN) is much more powerful than the fragment address' on the end of some URLs, as it allows you to specify the location of a link end using the structure of the document as well as (or in addition to) known, fixed points like IDs. For example, the word XPointer' two paragraphs back could be referred to as http://www.ucc.ie/xml/faq.sgml#ID(faq-hypertext)CHILD(2,*)(6,*), meaning the sixth child object within the second child object after the element whose ID is faq-hypertext. Count the objects from the start of this question in the SGML version (which has the ID faq-hypertext'):

1. the title of the question;

<SECT2 ID="faq-hypertext">
<TITLE>How will XML affect my document links?</TITLE>
2. the second paragraph:

1. the character data from the start of the paragraph to the first item of markup:

<PARA>The
2. the markup item:

<ULINK URL="http://www.w3.org/TR/WD-xlink">XML Linking
Specification (XLink)</ULINK>
3. the text item:

and
4. the markup item:

<ULINK
URL="http://www.w3.org/TR/WD-xptr">XML Extended Pointer Specification
(XPointer)</ULINK>
5. the next stretch of character data:

documents contain a detailed specification. An XML link
can be either a URL or a TEI-style Extended Pointer (
6. and the next markup item:

<LINK LINKEND="tei-link">XPointer</LINK>

§ If you view this file with Panorama or MultiDoc Pro you can click on the highlighted cross-reference button at the start of the example sentence, and it will display the locations in Extended Pointer Notation of all the links to it, including the word XPointer' mentioned. (Doing this in an HTML browser is not meaningful, as they do not support bidirectional linking or EPN.) David Megginson has produced an additional function for Emacs/psgml which will deduce an XPointer for any location in an SGML or XML file.

C.11   Can I do mathematics using XML?

§ Yes, if the document type you use provides for math. The mathematics-using community is developing software, and there is a MathML proposal at the W3C, which is a native XML application. It would also be possible to make XML fragments from the long-expired HTML3, HTML Pro, or ISO 12083 Math, or OpenMath, or one of your own making. Browsers which display some math embedded in SGML already exist (eg DynaText, Panorama, Multidoc Pro).

§ The sophistication could vary from math expressions like xi through simple inline equations such as E = mc2 to display equations like

(b4iuru/16qtp )

If you are using an HTML browser to read this, of course, the above equations may not be rendered correctly. The Techexplorer plugin from IBM can be used with regular HTML browsers to render TeX math, and the Amaya testbed browser at the W3C has an experimental MathML display.

C.12   How does XML handle metadata?

Because XML lets you define your own markup language, you can make full use of the extended hypertext features (see the question on Links) of XML to store or link to metadata in any format (eg Dublin Core, Warwick Framework, Resource Description Framework (RDF), and Platform for Internet Content Selection (PICS)).

There are no predefined elements in XML, because it is an architecture, not an application, so it is not part of XML's job to specify how or if authors should or should not implement metadata. You are therefore free to use any suitable method from simple attributes to the embedding of entire Dublin Core/Warwick Framework metadata records. Browser makers may also have their own architectural recommendations or methods to propose.

C.13   Can I use Java, ActiveX, etc in XML?

This depends on what facilities the browser makers implement. XML is about describing information; scripting languages and languages for embedded functionality are the software which enables the information to be manipulated at the user's end.

XML itself provides a way to define the markup needed to implement scripting languages: as a neutral standard it neither encourages not discourages their use, and does not favour one language over another, so the field is wide open. Developments are ongoing: see John Tigue's suggestions for standardising the API for Java in respect of XML.

Scripting languages are provided for in a proposal for an Extensible Style Language, XSL (see question on Stylesheets).

C.14   How do I control appearance?

The use of a stylesheet is implicit in XML. Some browsers may possibly provide simple default styles for popular elements like <PARA>, or <LIST> containing <ITEM>, but in general a stylesheet gives the author much better control of the layout. But as with any system where files can be viewed at random by arbitrary users, the author cannot know what resources (such as fonts) are on the user's system, so care is needed.

There are also many pre-existing proprietary stylesheet systems and implementations, many of which are deeply embedded in the technical documentation community (and thus heavily supported by one or more products):

• The Synex stylesheet DTD as already used in Panorama and MultiDoc Pro;

• Inso Corp's DynaText and DynaWeb browser and server products have their own stylesheet language;

• The US military standard FOSI (Formatted Output Specification Instance) is implemented in Arbortext's ADEPT·Editor;

• SoftQuad's Author/Editor uses another separate stylesheet system.

It remains to be seen which ones browsers will implement.

C.15   How do I use graphics in XML?

§ Graphics are just links, so they can be done in any way supported by the XLink and XPointer specifications (see earlier question), including using similar syntax to existing HTML images, or done by using XML's built-in NOTATION and ENTITY mechanism. The linking specifications, however, give you much better control over the traversal and activation of links, so you can choose, for example, whether or not to have an image appear when the page is loaded, or on a click from the user, or in a separate window, without having to resort to scripting. Which graphic file formats will be supported is a matter for the browser makers: XML itself doesn't predict or restrict you. GIF, JPG, TIFF, and CGM at a minimum would seem to make sense.

Peter Murray-Rust writes: GIFs and JPegs cater for bitmaps (pixel representations of images). Vector graphics (scaleable) are being addressed in the W3C's graphics activity (see http://www.w3.org/Graphics/Activity). When a consensus is reached it will be possible to transmit the graphics representation within the XML file. For many graphics objects this will mean greatly decreased download time and scaling without loss of detail.

Bob DuCharme adds: All the data in an XML document entity must be parseable XML. You can define an external entity as either a parsed entity (parseable XML) or an unparsed entity (anything else). Unparsed entities can be used for picture files, sound files, movie files, or whatever you like. They can only be referenced from within a document as the value of an attribute (much like a bitmap picture on an HTML Web page is the value of the <img> element's src attribute) and not part of the actual document. In an XML document, this attribute must be declared to be of type ENTITY, and the entity's declaration must specify a declared NOTATION, because if the entity isn't XML, the XML processor needs to know what it is. For example, in the following document, the colliepic entity is declared to have a JPEG notation, and it's used as the value of the empty dog element's picfile attribute.

<?xml version="1.0"?>
<!DOCTYPE dog [
<!NOTATION JPEG SYSTEM "Joint Photographic Experts Group">
<!ENTITY colliepic SYSTEM "lassie.jpg" NDATA JPEG>
<!ELEMENT dog EMPTY>
<!ATTLIST dog picfile ENTITY #REQUIRED>
]>
<dog picfile="colliepic"/>

The XLink and XPointer linking specifications describe other ways to point to a non-XML file such as a graphic. These offer more sophisticated control over the external entity's position, handling, and appearance within the XML document.

D. Developers and Implementors (including WebMasters and server operators)

D.1   Where's the spec?

Right here (http://www.w3.org/TR/REC-xml). Includes the EBNF. There are also versions in Japanese (http://www.fxis.co.jp/DMS/sgml/xml/); Spanish (http://www.ucc.ie/xml/faq-es.html); Korean (http://xml.t2000.co.kr/faq/index.html) and a Java-ised annotated version at http://www.xml.com/axml/testaxml.htm.

Eve Maler has released the DTD and documentation used for the spec itself: this is a new version that was used to encode the XML, XLink, XPointer, DOM, etc specifications. Be aware that this version is no longer compatible with the version that XML 1.0 uses; please send any comments or questions to Eve.

D.2   What are these terms DTDless', valid', and well-formed'?

Full SGML uses a Document Type Definition (DTD) to describe the markup (elements) available in any specific type of document. However, the design and construction of a DTD can be a complex and non-trivial task, so XML has been designed so it can be used either with or without a DTD. DTDless operation means you can invent markup without having to define it formally.

To make this work, a DTDless file in effect defines' its own markup, informally, by the existence and location of elements where you create them. But when an XML application such as a browser encounters a DTDless file, it needs to be able to understand the document structure as it reads it, because it has no DTD to tell it what to expect, so some changes have been made to the rules.

For example, HTML's <IMG> element is defined as EMPTY': it doesn't have an end-tag. Without a DTD, an XML application would have no way to know whether or not to expect an end-tag for an element, so the concept of well-formed' has been introduced. This makes the start and end of every element, and the occurrence of EMPTY elements completely unambiguous.

D.2.1   Well-formed' documents

All XML documents must be well-formed:

• if there is no DTD in use, the document must start with a Standalone Document Declaration (SDD) saying so:

<?xml version="1.0" standalone="yes"?>
<foo>
<bar>...<blort/>...</bar>
</foo>

• all tags must be balanced: that is, all elements which may contain character data must have both start- and end-tags present (omission is not allowed except for empty elements, see below);

• all attribute values must be in quotes (the single-quote character [the apostrophe] may be used if the value contains a double-quote character, and vice versa): if you need both, use &apos; and &quot;

• any EMPTY element tags (eg those with no end-tag like HTML's <IMG>, <HR>, and <BR> and others) must either end with />' or you have to make them non-EMPTY by adding a real end-tag;

Example: <BR> would become either <BR/> or <BR></BR>.

• there must not be any isolated markup characters (< or &) in your text data (ie they must be given as &lt; and &amp;), and the sequence ]]> must be given as ]]&gt; if it does not occur as the end of a CDATA marked section;

• elements must nest inside each other properly (no overlapping markup, same rule as for all SGML);

• Well-formed files with no DTD may use attributes on any element, but the attributes must all be of type CDATA by default.

§ Well-formed XML files with no DTD are considered to have &lt;, &gt;, &apos;, &quot;, and &amp; predefined and thus available for use even without a DTD. Valid XML files must declare them explicitly if they use them. Richard Lander notes that it is possible to declare character entities in the internal subset of an otherwise DTDless file and still remain standalone if you want to use more than the five default character entities above but avoid having to write a DTD:

<?xml version="1.0" standalone="yes"?>
<!doctype example [
<!entity nbsp "&#032;">
]>
<example>
<!-- the rest goes here -->
</example>

D.2.2   Valid XML

Valid XML files are those which have a Document Type Definition (DTD) like all other SGML applications, and which adhere to it. They must also be well-formed.

A valid file begins like any other SGML file with a Document Type Declaration, but may have an optional XML Declaration prepended:

<?xml version="1.0"?>
<text>...</text>
</advert>

The XML Specification defines an SGML Declaration for XML which is fixed for all instances (the declaration has been removed from the text of the Specification and is now in a separate document). An XML version of the specified DTD must be accessible to the XML processor, either by being available locally (ie the user already has a copy on disk), or by being retrievable via the network. You can specify this by supplying the URL for the DTD in a System Identifier (as in the example above). It is possible (some people would say preferable) to supply a Formal Public Identifier, but if used, this must precede the System Identifier, which must still be given (and only the PUBLIC keyword is used), eg:

<!DOCTYPE advert PUBLIC "-//Foo, Inc//DTD Advertisements//EN" "http://www.foo.org/ad.dtd">

or

<!DOCTYPE advert SYSTEM "http://www.foo.org/ad.dtd">

The defaults for the other attributes of the XML Declaration are VERSION="1.0" and ENCODING="UTF-8".

D.3   What else has changed between SGML and XML?

§ The principal changes are in what you can do in writing a Document Type Definition (DTD). To simplify the syntax and make it easier to write processing software, a large number of SGML markup declaration options have been suppressed (see the reference to the list of omitted features).

§ An extra delimiter is permitted in Names (the colon) for use in experiments with namespaces (enabling DTDs to distinguish element source, ownership, or application). A colon may only appear in mid-name, though, not at the start or the end. Work is ongoing to define how these can be declared and referenced using element and attribute markup.

D.4   What XML software can I use today?

Details have been removed as they are now changing too rapidly to be duplicated in this FAQ: see the XML pages at http://www.oasis-open.org/cover/xml.html.

For browsers see the question on XML Browsers and the details of the xml-dev mailing list for software developers. Bert Bos keeps a list of some XML developments in bison, flex, perl and Python.

D.5   Do I have to change any of my server software to work with XML?

Only to serve up .xml files as the correct MIME type (application/xml or text/xml, RFC2376), so for serving XML documents all that is needed is to edit the mime-types file (or its equivalent) and add the line

application/xml            xml XML dtd DTD xsl XSL ent ENT cat CAT sty STY

In some servers (eg Apache), users can change the MIME type for specific file types from their own directories by using a .htaccess file. The MIME content-type text/xml can only be applied to pure ASCII files (ISO 646 IRV) because of a character-set restriction in the RFC.

Since XML is designed to support stylesheets and sophisticated hyperlinking, XML documents will be accompanied by ancillary files in the same way that SGML files are: DTDs, entity files, catalogs, stylesheets, etc, which may need other MIME Content-Type entries, and which require placing in the appropriate directories. XUA (XML User Agent), which is one of the planned deliverables of the XML WG, might provide a mechanism for packaging XML documents and XSL styles into a single message.

If you run scripts generating HTML, which you wish to work with XML, they will need to be modified to produce the relevant document type.

D.6   Can I still use server-side INCLUDEs?

Yes, so long as what they generate ends up as part of an XML-conformant file (ie either valid or just well-formed).

D.7   Can I (and my authors) still use client-side INCLUDEs?

The same rule applies as for server-side INCLUDEs, so you need to ensure that any embedded code which gets passed to a third-party engine (eg SDQL enquiries, Java writes, LiveWire requests, streamed content, etc) does not contain any characters which might be misinterpreted as XML markup (ie no angle brackets or ampersands): either use a CDATA marked section to avoid your XML application parsing the embedded code, or use the standard &lt;, &gt;, and &amp; character entity references instead.

D.8   I'm trying to understand the XML Spec: why does SGML (and XML) have such difficult terminology?

For implementation to succeed, the terminology needs to be precise.

Example: element' and tag' are not synonymous: an element is a whole unit of information with its markup, and may consist of a start-tag alone (as in HTML's <BR>) or a start-tag and an end-tag and the content which goes between them; tags alone are simply the markers at the start and end of elements.

Sloppy terminology in specifications causes misunderstandings, so formal standards have to be phrased in formal terminology. This is not a formal document, and the astute reader may already have noticed it refers to element names' where element type names' is more correct; but the former is more widely understood.

Those new to SGML may want to read something like the Gentle Introduction to SGML chapter of the TEI.

D.9   Is there a Developer's API kit for XML?

Several are available or under development. Details of these and other XML software are held on the SGML/XML Web Pages at http://

The big SGML conversion and application development engines like Balise, Omnimark, and SGMLC are all working on XML versions. Details of SGML software of all kinds are on the SGML Web pages.

D.10   How does XML fit with the DOM?

Michael Dyck writes: the Document Object Model (DOM) (http://www.w3.org/TR/PR-DOM-Level-1) provides an abstract API for constructing, accessing, and manipulating XML and HTML documents. A binding' of the DOM to a particular programming language provides a concrete API.

D.11   Is there a conformance test suite for XML processors?

James Clark has a collection of test cases for testing XML parsers at http://www.jclark.com/xml/ which includes a conformance test.

D.12   How do I include one DTD (or fragment) in another?

This works exactly the same as for regular SGML. First you declare the entity you want to include, and then you reference it by name:

<!ENTITY % mylists PUBLIC
"-//Foo, Inc//ENTITIES Common list structures//EN"
"dtds/listfrag.ent">
...
%mylists;

Such declarations tranditionally go all together at the top of the main DTD file, where they can be managed and maintained, but this is not essential. You use parameter entity syntax for this (the percent sign) because the file is to be included at DTD compile time, not when the document instance is parsed.

Note that a URL is compulsory in XML for all external file references: standard rules for dereferencing URLs apply (assume the same method, server, and directory as the containing document). The URL can be supplied either as a System Identifier alone:

<!ENTITY mydtd SYSTEM "http://www.foo.bar/~blort/my.dtd">

or as a second parameter to a formal Public Identifier as in the earlier example.

D.13   I've already got SGML DTDs: how do I convert them for use with XML?

There are numerous projects being started to convert common or popular SGML DTDs to XML format (for example Patrice Bonhomme is working on an unofficial XML version of the TEI Lite DTD: details of that are discussed on the TEI-L mailing list).

The following checklist comes courtesy of Seán McGrath (author of XML By Example):

1. No equivalent of the SGML Declaration. So keywords, character set etc are essentially fixed;

2. Tag mimimization is not allowed, so <!ELEMENT x - O (A,B)> becomes <!ELEMENT X (A,B)> and <!ELEMENT x - O EMPTY> becomes <!ELEMENT X EMPTY>;

3. #PCDATA must only occur extreme left in an OR model, eg<!ELEMENT x (A|B|#PCDATA|C)> becomes <!ELEMENT x (#PCDATA|A|B|C)> and <!ELEMENT x (A,#PCDATA)> is illegal;

4. No CDATA, RCDATA elements;

5. Some SGML attribute types are not allowed in XML eg NUTOKEN. Also there are no NOTATION attributes (data attributes);

6. Some SGML attribute defaults are not allowed in XML eg CONREF;

7. Comments cannot be inline to declarations like <!ELEMENT x (A,B) -- this is an SGML comment in a declaration -->;

8. A whole bunch of SGML optional features are not present in XML: a) all forms of tag minimization (OMITTAG, DATATAG, SHORTREF, etc); b) Link Process Definitions; c) Multiple DTDs per document and many more: see the question on the bits of SGML that were removed for XML for a reference to the complete list;

9. And last but not least, CONCUR! There are some important differences betweeen the internal and external subset portion of a DTD in XML: a) marked sections can only occur in the external subset b) Parameter Entities must be used to replace entire declarations in the internal subset portion of a DTD, eg this is invalid XML:

<!DOCTYPE x [
<!ENTITY % modelx "(A|B)*">
<!ELEMENT x %modelx;>
]>
<x></x>

D.14   What's the story on XML and EDI?

Electronic Document Interchange has been used for several years to exchange documents between commercial partners to a transaction. It has required special proprietary software, but there are now moves to enable EDI data to travel inside XML. Details of developments are at http://www.xmledi.com/ and there is a guideline document at http://www.geocities.com/WallStreet/Floor/5815/guide.htm.

Response and query form

Illustration from Dale Dougherty's article in Web Review (courtesy of the publishers).