XML

Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification^[4] produced by the W3C, and several other related specifications, all gratis open standards.^[5]
The design goals of XML emphasize simplicity, generality, and usability over the Internet.^[6] It is a textual data format with strong support via Unicode for the languages of the world. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.
Many application programming interfaces (APIs) have been developed that software developers use to process XML data, and several schema systems exist to aid in the definition of XML-based languages.
As of 2009, hundreds of XML-based languages have been developed,^[7] including RSS, Atom, SOAP, and XHTML. XML-based formats have become the default for most office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org (OpenDocument), and Apple's iWork.^[8]

Contents [hide] 1 Key terminology 2 Characters and escaping 2.1 Valid characters 2.2 Encoding detection 2.3 Escaping 2.4 Comments 2.5 International use 3 Well-formedness and error-handling 4 Schemas and validation 4.1 DTD 4.2 XML Schema 4.3 RELAX NG 4.4 Schematron 4.5 ISO DSDL and other schema languages 5 Related specifications 6 Use on the Internet 7 Programming interfaces 7.1 Simple API for XML (SAX) 7.2 Pull parsing 7.3 Document Object Model (DOM) 7.4 Data binding 7.5 XML as data type 8 History 8.1 Sources 8.2 Versions 9 Criticism 10 See also 11 References 12 Further reading 13 External links

[edit] Key terminology

The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs which appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use.

(Unicode) Character

By definition, an XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.

Processor and Application

The processor analyzes the markup and passes structured information to an application. The specification places requirements on what an XML processor must do and not do, but the application is outside its scope. The processor (as the specification calls it) is often referred to colloquially as an XML parser.

Markup and Content

The characters which make up an XML document are divided into markup and content. Markup and content may be distinguished by the application of simple syntactic rules. All strings which constitute markup either begin with the character "<" and end with a ">", or begin with the character "&" and end with a ";". Strings of characters which are not markup are content.

Tag

A markup construct that begins with "<" and ends with ">". Tags come in three flavors: start-tags, for example <section>, end-tags, for example </section>, and empty-element tags, for example <line-break />.

Element

A logical document component that either begins with a start-tag and ends with a matching end-tag or consists only of an empty-element tag. The characters between the start- and end-tags, if any, are the element's content, and may contain markup, including other elements, which are called child elements. An example of an element is <Greeting>Hello, world.</Greeting> (see hello world). Another is <line-break />.

Attribute

A markup construct consisting of a name/value pair that exists within a start-tag or empty-element tag. In the example (below) the element img has two attributes, src and alt: <img src="madonna.jpg" alt='Foligno Madonna, by Raphael' />. Another example would be <step number="3">Connect A to B.</step> where the name of the attribute is "number" and the value is "3".

XML Declaration

XML documents may begin by declaring some information about themselves, as in the following example.

<?xml version="1.0" encoding="UTF-8" ?>

[edit] Characters and escaping

XML documents consist entirely of characters from the Unicode repertoire. Except for a small number of specifically excluded control characters, any character defined by Unicode may appear within the content of an XML document. The selection of characters that may appear within markup is somewhat more limited but still large.
XML includes facilities for identifying the encoding of the Unicode characters that make up the document, and for expressing characters that, for one reason or another, cannot be used directly.

[edit] Valid characters

Main article: Valid characters in XML

Unicode code points in the following ranges are valid in XML 1.0 documents:^[9]

• U+0009, U+000A, U+000D: these are the only C0 controls accepted in XML 1.0;
• U+0020–U+D7FF, U+E000–U+FFFD: this excludes some (not all) non-characters in the BMP (all surrogates, U+FFFE and U+FFFF are forbidden);
• U+10000–U+10FFFF: this includes all code points in supplementary planes, including non-characters.

XML 1.1^[10] extends the set of allowed characters to include all the above, plus the remaining characters in the range U+0001–U+001F. At the same time, however, it restricts the use of C0 and C1 control characters other than U+0009, U+000A, U+000D, and U+0085 by requiring them to be written in escaped form (for example U+0001 must be written as &#x01; or its equivalent). In the case of C1 characters, this restriction is a backwards incompatibility; it was introduced to allow common encoding errors to be detected.
The code point U+0000 is the only character that is not permitted in any XML 1.0 or 1.1 document.

[edit] Encoding detection

The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings". Unicode itself defines encodings that cover the entire repertoire; well-known ones include UTF-8 and UTF-16.^[11] There are many other text encodings that pre-date Unicode, such as ASCII and ISO/IEC 8859; their character repertoires in almost every case are subsets of the Unicode character set.
XML allows the use of any of the Unicode-defined encodings, and any other encodings whose characters also appear in Unicode. XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used.^[12] Encodings other than UTF-8 and UTF-16 will not necessarily be recognized by every XML parser.

[edit] Escaping

XML provides escape facilities for including characters which are problematic to include directly. For example:

• The characters "<" and "&" are key syntax markers and may never appear in content outside of a CDATA section.^[13]
• Some character encodings support only a subset of Unicode: for example, it is legal to encode an XML document in ASCII, but ASCII lacks code points for Unicode characters such as "é".
• It might not be possible to type the character on the author's machine.
• Some characters have glyphs that cannot be visually distinguished from other characters: examples are non-breaking-space ( ) and Cyrillic Capital Letter A (А).

There are five predefined entities: < represents "<", > represents ">", & represents "&", ' represents ', and " represents ". All permitted Unicode characters may be represented with a numeric character reference. Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as 中 or 中. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as "I <3 Jörg".
"�" is not permitted, however, as the null character is one of the control characters excluded from XML, even when using a numeric character reference.^[14] An alternative encoding mechanism such as Base64 is needed to represent such characters.

[edit]

Comments may appear anywhere in a document outside other markup. Comments cannot appear before the XML declaration. The string "--" (double-hyphen) is not allowed inside comments. Comments start with "<!--". The ampersand has no special significance within comments, so entity and character references are not recognized as such, and there is no way to represent characters outside the character set of the document encoding.
An example of a valid comment: "<!-- no need to escape <code> & such in comments -->"

[edit] International use

This example contains Chinese text. Without proper rendering support, you may see question marks, boxes, or other symbols instead of Chinese characters.

This example contains Cyrillic text. Without proper rendering support, you may see question marks or boxes, misplaced vowels or missing conjuncts instead of Cyrillic letters.

XML supports the direct use of almost any Unicode character in element names, attributes, comments, character data, and processing instructions (other than the ones that have special symbolic meaning in XML itself, such as the less-than sign, "<"). Therefore, the following is a well-formed XML document, even though it includes both Chinese and Cyrillic characters:

<?xml version="1.0" encoding="UTF-8" ?>
<俄语>Китайська мова</俄语>

[edit] Well-formedness and error-handling

Main article: Well-formed document

The XML specification defines an XML document as a text that is well-formed, i.e. it satisfies a list of syntax rules provided in the specification. The list is fairly lengthy; some key points are:

• It contains only properly encoded legal Unicode characters.
• None of the special syntax characters such as "<" and "&" appear except when performing their markup-delineation roles.
• The begin, end, and empty-element tags that delimit the elements are correctly nested, with none missing and none overlapping.
• The element tags are case-sensitive; the beginning and end tags must match exactly. Tag names cannot contain any of the characters !"#$%&'()*+,/;<=>?@[\]^`{|}~, nor a space character, and cannot start with -, ., or a numeric digit.
• There is a single "root" element that contains all the other elements.

The definition of an XML document excludes texts that contain violations of well-formedness rules; they are simply not XML. An XML processor that encounters such a violation is required to report such errors and to cease normal processing. This policy, occasionally referred to as draconian, stands in notable contrast to the behavior of programs that process HTML, which are designed to produce a reasonable result even in the presence of severe markup errors. XML's policy in this area has been criticized as a violation of Postel's law.^[15]

[edit] Schemas and validation

In addition to being well-formed, an XML document may be valid. This means that it contains a reference to a Document Type Definition (DTD), and that its elements and attributes are declared in that DTD and follow the grammatical rules for them that the DTD specifies.
XML processors are classified as validating or non-validating depending on whether or not they check XML documents for validity. A processor that discovers a validity error must be able to report it, but may continue normal processing.
A DTD is an example of a schema or grammar. Since the initial publication of XML 1.0, there has been substantial work in the area of schema languages for XML. Such schema languages typically constrain the set of elements that may be used in a document, which attributes may be applied to them, the order in which they may appear, and the allowable parent/child relationships.

[edit] DTD

Main article: Document Type Definition

The oldest schema language for XML is the Document Type Definition (DTD), inherited from SGML.
DTDs have the following benefits:

• DTD support is ubiquitous due to its inclusion in the XML 1.0 standard.
• DTDs are terse compared to element-based schema languages and consequently present more information in a single screen.
• DTDs allow the declaration of standard public entity sets for publishing characters.
• DTDs define a document type rather than the types used by a namespace, thus grouping all constraints for a document in a single collection.

DTDs have the following limitations:

• They have no explicit support for newer features of XML, most importantly namespaces.
• They lack expressiveness. XML DTDs are simpler than SGML DTDs and there are certain structures that cannot be expressed with regular grammars. DTDs only support rudimentary datatypes.
• They lack readability. DTD designers typically make heavy use of parameter entities (which behave essentially as textual macros), which make it easier to define complex grammars, but at the expense of clarity.
• They use a syntax based on regular expression syntax, inherited from SGML, to describe the schema. Typical XML APIs such as SAX do not attempt to offer applications a structured representation of the syntax, so it is less accessible to programmers than an element-based syntax may be.

Two peculiar features that distinguish DTDs from other schema types are the syntactic support for embedding a DTD within XML documents and for defining entities, which are arbitrary fragments of text and/or markup that the XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes.
DTD technology is still used in many applications because of its ubiquity.

[edit] XML Schema

Main article: XML Schema (W3C)

A newer schema language, described by the W3C as the successor of DTDs, is XML Schema, often referred to by the initialism for XML Schema instances, XSD (XML Schema Definition). XSDs are far more powerful than DTDs in describing XML languages. They use a rich datatyping system and allow for more detailed constraints on an XML document's logical structure. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them.

[edit] RELAX NG

Main article: RELAX NG

RELAX NG was initially specified by OASIS and is now also an ISO international standard (as part of DSDL). RELAX NG schemas may be written in either an XML based syntax or a more compact non-XML syntax; the two syntaxes are isomorphic and James Clark's Trang conversion tool can convert between them without loss of information. RELAX NG has a simpler definition and validation framework than XML Schema, making it easier to use and implement. It also has the ability to use datatype framework plug-ins; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes.

[edit] Schematron

Main article: Schematron

Schematron is a language for making assertions about the presence or absence of patterns in an XML document. It typically uses XPath expressions.

[edit] ISO DSDL and other schema languages

The ISO DSDL (Document Schema Description Languages) standard brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes RELAX NG full and compact syntax, Schematron assertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based routing of document fragments to different validators. DSDL schema languages do not have the vendor support of XML Schemas yet, and are to some extent a grassroots reaction of industrial publishers to the lack of utility of XML Schemas for publishing.
Some schema languages not only describe the structure of a particular XML format but also offer limited facilities to influence processing of individual XML files that conform to this format. DTDs and XSDs both have this ability; they can for instance provide the infoset augmentation facility and attribute defaults. RELAX NG and Schematron intentionally do not provide these.

[edit] Related specifications

A cluster of specifications closely related to XML have been developed, starting soon after the initial publication of XML 1.0. It is frequently the case that the term "XML" is used to refer to XML together with one or more of these other technologies which have come to be seen as part of the XML core.

• XML Namespaces enable the same document to contain XML elements and attributes taken from different vocabularies, without any naming collisions occurring. Although XML Namespaces are not part of the XML specification itself, virtually all XML software also supports XML Namespaces.
• XML Base defines the xml:base attribute, which may be used to set the base for resolution of relative URI references within the scope of a single XML element.
• The XML Information Set or XML infoset describes an abstract data model for XML documents in terms of information items. The infoset is commonly used in the specifications of XML languages, for convenience in describing constraints on the XML constructs those languages allow.
• xml:id Version 1.0 asserts that an attribute named xml:id functions as an "ID attribute" in the sense used in a DTD.
• XPath defines a syntax named XPath expressions which identifies one or more of the internal components (elements, attributes, and so on) included in an XML document. XPath is widely used in other core-XML specifications and in programming libraries for accessing XML-encoded data.
• XSLT is a language with an XML-based syntax that is used to transform XML documents into other XML documents, HTML, or other, unstructured formats such as plain text or RTF. XSLT is very tightly coupled with XPath, which it uses to address components of the input XML document, mainly elements and attributes.
• XSL Formatting Objects, or XSL-FO, is a markup language for XML document formatting which is most often used to generate PDFs.
• XQuery is an XML-oriented query language strongly rooted in XPath and XML Schema. It provides methods to access, manipulate and return XML, and is mainly conceived as a query language for XML databases.
• XML Signature defines syntax and processing rules for creating digital signatures on XML content.
• XML Encryption defines syntax and processing rules for encrypting XML content.

Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including XInclude, XLink, and XPointer.

[edit] Use on the Internet

It is common for XML to be used in interchanging data over the Internet. RFC 3023 gives rules for the construction of Internet Media Types for use when sending XML. It also defines the types "application/xml" and "text/xml", which say only that the data is in XML, and nothing about its semantics. The use of "text/xml" has been criticized as a potential source of encoding problems and is now in the process of being deprecated.^[16] RFC 3023 also recommends that XML-based languages be given media types beginning in "application/" and ending in "+xml"; for example "application/svg+xml" for SVG.
Further guidelines for the use of XML in a networked context may be found in RFC 3470, also known as IETF BCP 70; this document is very wide-ranging and covers many aspects of designing and deploying an XML-based language.

[edit] Programming interfaces

The design goals of XML include "It shall be easy to write programs which process XML documents."^[6] Despite this fact, the XML specification contains almost no information about how programmers might go about doing such processing. The XML Infoset provides a vocabulary to refer to the constructs within an XML document, but once again does not provide any guidance on how to access this information. A variety of APIs for accessing XML have been developed and used, and some have been standardized.
Existing APIs for XML processing tend to fall into these categories:

• Stream-oriented APIs accessible from a programming language, for example SAX and StAX.
• Tree-traversal APIs accessible from a programming language, for example DOM.
• XML data binding, which provides an automated translation between an XML document and programming-language objects.
• Declarative transformation languages such as XSLT and XQuery.

Stream-oriented facilities require less memory and, for certain tasks which are based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and data-binding APIs typically require the use of much more memory, but are often found more convenient for use by programmers; some include declarative retrieval of document components via the use of XPath expressions.
XSLT is designed for declarative description of XML document transformations, and has been widely implemented both in server-side packages and Web browsers. XQuery overlaps XSLT in its functionality, but is designed more for searching of large XML databases.

[edit] Simple API for XML (SAX)

SAX is a lexical, event-driven interface in which a document is read serially and its contents are reported as callbacks to various methods on a handler object of the user's design. SAX is fast and efficient to implement, but difficult to use for extracting information at random from the XML, since it tends to burden the application author with keeping track of what part of the document is being processed. It is better suited to situations in which certain types of information are always handled the same way, no matter where they occur in the document.

[edit] Pull parsing

Pull parsing^[17] treats the document as a series of items which are read in sequence using the Iterator design pattern. This allows for writing of recursive-descent parsers in which the structure of the code performing the parsing mirrors the structure of the XML being parsed, and intermediate parsed results can be used and accessed as local variables within the methods performing the parsing, or passed down (as method parameters) into lower-level methods, or returned (as method return values) to higher-level methods. Examples of pull parsers include StAX in the Java programming language, XMLReader in PHP and System.Xml.XmlReader in the .NET Framework.
A pull parser creates an iterator that sequentially visits the various elements, attributes, and data in an XML document. Code which uses this iterator can test the current item (to tell, for example, whether it is a start or end element, or text), and inspect its attributes (local name, namespace, values of XML attributes, value of text, etc.), and can also move the iterator to the next item. The code can thus extract information from the document as it traverses it. The recursive-descent approach tends to lend itself to keeping data as typed local variables in the code doing the parsing, while SAX, for instance, typically requires a parser to manually maintain intermediate data within a stack of elements which are parent elements of the element being parsed. Pull-parsing code can be more straightforward to understand and maintain than SAX parsing code..

[edit] Document Object Model (DOM)

DOM (Document Object Model) is an interface-oriented Application Programming Interface that allows for navigation of the entire document as if it were a tree of "Node" objects representing the document's contents. A DOM document can be created by a parser, or can be generated manually by users (with limitations). Data types in DOM Nodes are abstract; implementations provide their own programming language-specific bindings. DOM implementations tend to be memory intensive, as they generally require the entire document to be loaded into memory and constructed as a tree of objects before access is allowed.

[edit] Data binding

Another form of XML processing API is XML data binding, where XML data is made available as a hierarchy of custom, strongly typed classes, in contrast to the generic objects created by a Document Object Model parser. This approach simplifies code development, and in many cases allows problems to be identified at compile time rather than run-time. Example data binding systems include the Java Architecture for XML Binding (JAXB) and XML Serialization in .NET.^[18]

[edit] XML as data type

XML is beginning to appear as a first-class data type in other languages. The ECMAScript for XML (E4X) extension to the ECMAScript/JavaScript language explicitly defines two specific objects (XML and XMLList) for JavaScript, which support XML document nodes and XML node lists as distinct objects and use a dot-notation specifying parent-child relationships.^[19] E4X is supported by the Mozilla 2.5+ browsers and Adobe Actionscript, but has not been adopted more universally. Similar notations are used in Microsoft's LINQ implementation for Microsoft .NET 3.5 and above, and in Scala (which uses the Java VM). The open-source xmlsh application, which provides a Linux-like shell with special features for XML manipulation, similarly treats XML as a data type, using the <[ ]> notation.^[20] The Resource Description Framework defines a data type rdf:XMLLiteral to hold wrapped, canonical XML.^[21]

[edit] History

XML is an application profile of SGML (ISO 8879).^[22]
The versatility of SGML for dynamic information display was understood by early digital media publishers in the late 1980s prior to the rise of the Internet.^[23][24] By the mid-1990s some practitioners of SGML had gained experience with the then-new World Wide Web, and believed that SGML offered solutions to some of the problems the Web was likely to face as it grew. Dan Connolly added SGML to the list of W3C's activities when he joined the staff in 1995; work began in mid-1996 when Sun Microsystems engineer Jon Bosak developed a charter and recruited collaborators. Bosak was well connected in the small community of people who had experience both in SGML and the Web.^[25]
XML was compiled by a working group of eleven members,^[26] supported by an (approximately) 150-member Interest Group. Technical debate took place on the Interest Group mailing list and issues were resolved by consensus or, when that failed, majority vote of the Working Group. A record of design decisions and their rationales was compiled by Michael Sperberg-McQueen on December 4, 1997.^[27] James Clark served as Technical Lead of the Working Group, notably contributing the empty-element "<empty />" syntax and the name "XML". Other names that had been put forward for consideration included "MAGMA" (Minimal Architecture for Generalized Markup Applications), "SLIM" (Structured Language for Internet Markup) and "MGML" (Minimal Generalized Markup Language). The co-editors of the specification were originally Tim Bray and Michael Sperberg-McQueen. Halfway through the project Bray accepted a consulting engagement with Netscape, provoking vociferous protests from Microsoft. Bray was temporarily asked to resign the editorship. This led to intense dispute in the Working Group, eventually solved by the appointment of Microsoft's Jean Paoli as a third co-editor.
The XML Working Group never met face-to-face; the design was accomplished using a combination of email and weekly teleconferences. The major design decisions were reached in twenty weeks of intense work between July and November 1996, when the first Working Draft of an XML specification was published.^[28] Further design work continued through 1997, and XML 1.0 became a W3C Recommendation on February 10, 1998.

[edit] Sources

XML is a profile of an ISO standard SGML, and most of XML comes from SGML unchanged. From SGML comes the separation of logical and physical structures (elements and entities), the availability of grammar-based validation (DTDs), the separation of data and metadata (elements and attributes), mixed content, the separation of processing from representation (processing instructions), and the default angle-bracket syntax. Removed were the SGML Declaration (XML has a fixed delimiter set and adopts Unicode as the document character set).
Other sources of technology for XML were the Text Encoding Initiative (TEI), which defined a profile of SGML for use as a "transfer syntax"; and HTML, in which elements were synchronous with their resource, document character sets were separate from resource encoding, the xml:lang attribute was invented, and (like HTTP) metadata accompanied the resource rather than being needed at the declaration of a link. The Extended Reference Concrete Syntax (ERCS) project of the SPREAD (Standardization Project Regarding East Asian Documents) project of the ISO-related China/Japan/Korea Document Processing expert group was the basis of XML 1.0's naming rules; SPREAD also introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters. To support ERCS, XML and HTML better, the SGML standard IS 8879 was revised in 1996 and 1998 with WebSGML Adaptations. The XML header followed that of ISO HyTime.
Ideas that developed during discussion which were novel in XML included the algorithm for encoding detection and the encoding header, the processing instruction target, the xml:space attribute, and the new close delimiter for empty-element tags. The notion of well-formedness as opposed to validity (which enables parsing without a schema) was first formalized in XML, although it had been implemented successfully in the Electronic Book Technology "Dynatext" software;^[29] the software from the University of Waterloo New Oxford English Dictionary Project; the RISP LISP SGML text processor at Uniscope, Tokyo; the US Army Missile Command IADS hypertext system; Mentor Graphics Context; Interleaf and Xerox Publishing System.

[edit] Versions

There are two current versions of XML. The first (XML 1.0) was initially defined in 1998. It has undergone minor revisions since then, without being given a new version number, and is currently in its fifth edition, as published on November 26, 2008. It is widely implemented and still recommended for general use.
The second (XML 1.1) was initially published on February 4, 2004, the same day as XML 1.0 Third Edition,^[30] and is currently in its second edition, as published on August 16, 2006. It contains features (some contentious) that are intended to make XML easier to use in certain cases.^[31] The main changes are to enable the use of line-ending characters used on EBCDIC platforms, and the use of scripts and characters absent from Unicode 3.2. XML 1.1 is not very widely implemented and is recommended for use only by those who need its unique features.^[32]
Prior to its fifth edition release, XML 1.0 differed from XML 1.1 in having stricter requirements for characters available for use in element and attribute names and unique identifiers: in the first four editions of XML 1.0 the characters were exclusively enumerated using a specific version of the Unicode standard (Unicode 2.0 to Unicode 3.2.) The fifth edition substitutes the mechanism of XML 1.1, which is more future-proof but reduces redundancy. The approach taken in the fifth edition of XML 1.0 and in all editions of XML 1.1 is that only certain characters are forbidden in names, and everything else is allowed, in order to accommodate the use of suitable name characters in future versions of Unicode. In the fifth edition, XML names may contain characters in the Balinese, Cham, or Phoenician scripts among many others which have been added to Unicode since Unicode 3.2.^[31]
Almost any Unicode code point can be used in the character data and attribute values of an XML 1.0 or 1.1 document, even if the character corresponding to the code point is not defined in the current version of Unicode. In character data and attribute values, XML 1.1 allows the use of more control characters than XML 1.0, but, for "robustness", most of the control characters introduced in XML 1.1 must be expressed as numeric character references (and #x7F through #x9F, which had been allowed in XML 1.0, are in XML 1.1 even required to be expressed as numeric character references^[33]). Among the supported control characters in XML 1.1 are two line break codes that must be treated as whitespace. Whitespace characters are the only control codes that can be written directly.
There has been discussion of an XML 2.0, although no organization has announced plans for work on such a project. XML-SW (SW for skunkworks), written by one of the original developers of XML,^[34] contains some proposals for what an XML 2.0 might look like: elimination of DTDs from syntax, integration of namespaces, XML Base and XML Information Set (infoset) into the base standard.
The World Wide Web Consortium also has an XML Binary Characterization Working Group doing preliminary research into use cases and properties for a binary encoding of the XML infoset. The working group is not chartered to produce any official standards. Since XML is by definition text-based, ITU-T and ISO are using the name Fast Infoset for their own binary infoset to avoid confusion (see ITU-T Rec. X.891 | ISO/IEC 24824-1).

[edit] Criticism

XML and its extensions have regularly been criticized for verbosity and complexity.^[35] Mapping the basic tree model of XML to type systems of programming languages or databases can be difficult, especially when XML is used for exchanging highly structured data between applications, which was not its primary design goal. Other criticisms attempt to refute the claim that XML is a self-describing language^[36] (though the XML specification itself makes no such claim). JSON and YAML are frequently proposed as alternatives; both focus on representing structured data, rather than narrative documents.

XHTML

From Wikipedia, the free encyclopedia

XHTML

Filename extension	.xhtml, .xht, .xml, .html, .htm
Internet media type	application/xhtml+xml
Developed by	World Wide Web Consortium
Initial release	26 January 2000
Latest release	1.1 (Second Edition) / 23 November 2010; 7 months ago
Type of format	Markup language
Extended from	XML, HTML
Standard(s)	1.0 (Recommendation), 1.0 SE (Recommendation), 1.1 (Recommendation), 1.1 SE (Recommendation), 5 (Working Draft)

HTML
HTML and HTML5 Dynamic HTML XHTML XHTML Mobile Profile and C-HTML Canvas element Character encodings Document Object Model Font family HTML editor HTML element HTML Frames HTML5 video HTML scripting Web browser engine Quirks mode Style sheets Unicode and HTML W3C and WHATWG Web colors Web Storage Comparison of document markup languages web browsers layout engines for HTML HTML5 HTML5 Canvas HTML5 Media Non-standard HTML XHTML (1.1)
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XHTML (eXtensible HyperText Markup Language) is a family of XML markup languages that mirror or extend versions of the widely-used Hypertext Markup Language (HTML), the language in which web pages are written.
While HTML (prior to HTML5) was defined as an application of Standard Generalized Markup Language (SGML), a very flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. Because XHTML documents need to be well-formed, they can be parsed using standard XML parsers—unlike HTML, which requires a lenient HTML-specific parser.
XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C Recommendation on May 31, 2001. XHTML5 is undergoing development as of September 2009, as part of the HTML5 specification.

Contents [hide] 1 Overview 1.1 Motivation 1.2 Relationship to HTML 1.3 Adoption 1.4 Criticism 2 Versions of XHTML 2.1 XHTML 1.0 2.2 Modularization of XHTML 2.3 XHTML 1.1—Module-based XHTML 2.4 XHTML Basic and XHTML-MP 2.5 XHTML 1.2 2.6 XHTML 2.0 2.7 XHTML5 3 Semantic content in XHTML 4 Valid XHTML documents 4.1 Root element 4.2 DOCTYPEs 4.3 XML declaration 4.4 Common errors 5 Backward compatibility 5.1 Examples 6 Cross-compatibility of XHTML and HTML 7 See also 8 Notes 9 External links

[edit] Overview

XHTML 1.0 is "a reformulation of the three HTML 4 document types as applications of XML 1.0".^[1] The World Wide Web Consortium (W3C) also continues to maintain the HTML 4.01 Recommendation, and the specifications for HTML5 and XHTML5 are being actively developed. In the current XHTML 1.0 Recommendation document, as published and revised to August 2002, the W3C commented that, "The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while still remaining confident in their content's backward and future compatibility."^[1]
However, in 2004, the Web Hypertext Application Technology Working Group (WHATWG) formed, independently of the W3C, to work on advancing ordinary HTML not based on XHTML. Most major browser vendors were unwilling to implement the features in new W3C XHTML 2.0 drafts, and felt that they didn't serve the needs of modern web development.^{[citation needed]} The WHATWG eventually began working on a standard that supported both XML and non-XML serializations, HTML5, in parallel to W3C standards such as XHTML 2. In 2007, the W3C's HTML working group voted to officially recognize HTML5 and work on it as the next-generated HTML standard.^[2] In 2009, the W3C allowed the XHTML 2 Working Group's charter to expire, acknowledging that HTML5 would be the sole next-generation HTML standard, including both XML and non-XML serializations.^[3] Of the two serializations, the W3C suggests that most authors use the HTML syntax, rather than the XHTML syntax.^[4]

[edit] Motivation

XHTML was developed to make HTML more extensible and increase interoperability with other data formats.^[5] HTML 4 was ostensibly an application of Standard Generalized Markup Language (SGML); however the specification for SGML was complex, and neither web browsers nor the HTML 4 Recommendation were fully conformant to it.^[6] The XML standard, approved in 1998, provided a simpler data format closer in simplicity to HTML 4.^[7] By shifting to an XML format, it was hoped HTML would become compatible with common XML tools;^[8] servers and proxies would be able to transform content, as necessary, for constrained devices such as mobile phones.^[9] By utilizing namespaces, XHTML documents could provide extensibility by including fragments from other XML-based languages such as Scalable Vector Graphics and MathML.^[10] Finally, the renewed work would provide an opportunity to divide HTML into reusable components (XHTML Modularization) and clean up untidy parts of the language.^[11]

[edit] Relationship to HTML

There are various differences between XHTML and HTML. The Document Object Model is a tree structure which represents the page internally in applications, and XHTML and HTML are two different ways of representing that in markup (serialisations). Both are less expressive than the DOM (for example, "--" may be placed in comments in the DOM, but cannot be represented in a comment in either XHTML or HTML), and generally XHTML's XML syntax is a little more expressive than HTML (for example, arbitrary namespaces are not allowed in HTML). So, firstly one source of differences is immediate: XHTML uses an XML syntax, while HTML uses a pseudo-SGML syntax (officially SGML for HTML 4 and under, but never in practice, and standardised away from SGML in HTML5). Secondly however, because the expressible contents of the DOM in syntax are slightly different, there are some changes in actual behaviour between the two models.
Firstly then, syntax differences:^[12]

• Broadly, the XML rules require that all elements be closed, either by a separate closing tag or using self closing syntax (e.g. <br />), while HTML syntax permits some elements to be unclosed because either they are always empty (e.g. <input>) or their end can be determined implicitly ("omissibility", e.g. <p>).
• XML is case-sensitive for element and attribute names, while HTML is not.
• Some shorthand features in HTML are omitted in XML, such as (1) attribute minimization, where attribute values or their quotes may be omitted (e.g. <option selected> or <option selected=selected>, while XML this must be expressed as <option selected="selected">); (2) element minimization may be used to remove elements entirely (such as <tbody> inferred in a table if not given); and (3) the rarely used SGML syntax for element minimization ("shorttag"), which most browsers do not implement.
• There are numerous other technical requirements surrounding namespaces and precise parsing of whitespace and certain characters and elements. The exact parsing of HTML in practice has been undefined until recently; see the HTML5 specification ([HTML5]) for full details, or the working summary (HTML vs. XHTML).

Secondly, in contrast to these syntactical differences which are minor, there are some behavioural differences. Most of these arise from the underlying differences in serialisation. For example:

• Most prominently, behaviour on parse errors differ. A fatal parse error in XML (such as an incorrect tag structure) causes document processing to be aborted (a "yellow screen of death").
• Most content requiring namespaces will not work in HTML, except the built-in support for SVG and MathML in the HTML5 parser along with certain magic prefixes such as xlink.
• JavaScript processing is a little different, with minor changes in case sensitivity to some functions, and further precautions in XHTML to restrict processing to well-formed content. Third-party scripts, such as sidebars providing live feeds or Google's "web clips", often rely on the document.write() method, which is not available in XHTML pages so the scripts fail on pages served as XHTML (that is, using an XML MIME type). The innerHTML property is available on XML pages, but uses the same XML parser as the whole page, so will not insert content if a non-well-formed string is passed. More positively, innerHTML in XHTML can be used to insert namespaced content to the page.
• CSS is also applied slightly differently, with case-sensitivity and some differences in handling of backgrounds on <html> and <body> in XHTML and HTML.^[13]

[edit] Adoption

The similarities between HTML 4.01 and XHTML 1.0 led many web sites and content management systems to adopt the initial W3C XHTML 1.0 Recommendation. To aid authors in the transition, the W3C provided guidance on how to publish XHTML 1.0 documents in an HTML-compatible manner, and serve them to browsers that were not designed for XHTML.^[14][15]
Such "HTML-compatible" content is sent using the HTML media type (text/html) rather than the official Internet media type for XHTML (application/xhtml+xml). When measuring the adoption of XHTML to that of regular HTML, therefore, it is important to distinguish whether it is media type usage or actual document contents that is being compared.
Most web browsers have mature support^[16] for all of the possible XHTML media types.^[17] The notable exception is Internet Explorer versions 8 and earlier by Microsoft; rather than rendering application/xhtml+xml content, a dialog box invites the user to save the content to disk instead. Both Internet Explorer 7 (released in 2006) and Internet Explorer 8 (released in March 2009) exhibit this behavior.^[18] Microsoft developer Chris Wilson explained in 2005 that IE7’s priorities were improved security and CSS support, and that proper XHTML support would be difficult to graft onto IE’s compatibility-oriented HTML parser;^[19] however, Microsoft has added support for true XHTML in developer previews of IE9 that has carried over to the released version of IE9.^[20]
As long as support is not widespread, most web developers avoid using XHTML that isn’t HTML-compatible,^[21] so advantages of XML such as namespaces, faster parsing and smaller-footprint browsers do not benefit the user.

[edit] Criticism

In the early 2000s, some web developers began to question why Web authors ever made the leap into authoring in XHTML.^[22][23][24] Others countered that the problems ascribed to the use of XHTML could mostly be attributed to two main sources: the production of invalid XHTML documents by some Web authors and the lack of support for XHTML built into Internet Explorer 6.^[25][26] They went on to describe the benefits of XML-based Web documents (i.e. XHTML) regarding searching, indexing and parsing as well as future-proofing the Web itself.
In October 2006, HTML inventor and W3C chair Tim Berners-Lee, introducing a major W3C effort to develop new HTML specification, posted in his blog that, "The attempt to get the world to switch to XML … all at once didn't work. The large HTML-generating public did not move … Some large communities did shift and are enjoying the fruits of well-formed systems … The plan is to charter a completely new HTML group."^[27] The current HTML5 working draft says "special attention has been given to defining clear conformance criteria for user agents in an effort to improve interoperability … while at the same time updating the HTML specifications to address issues raised in the past few years." Ian Hickson, editor of the HTML5 specification criticising the improper use of XHTML in 2002,^[22] is a member of the group developing this specification and is listed as one of the co-editors of the current working draft.^[28]
Simon Pieters researched the XML-compliance of mobile browsers^[29] and concluded “the claim that XHTML would be needed for mobile devices is simply a myth”.

[edit] Versions of XHTML

[edit] XHTML 1.0

Wikipedia uses the XHTML 1.0 Transitional doctype and syntax, though it is not served as XHTML

December 1998 saw the publication of a W3C Working Draft entitled Reformulating HTML in XML. This introduced Voyager, the codename for a new markup language based on HTML 4 but adhering to the stricter syntax rules of XML. By February 1999 the specification had changed name to XHTML 1.0: The Extensible HyperText Markup Language, and in January 2000 it was officially adopted as a W3C Recommendation.^[30] There are three formal DTDs for XHTML 1.0, corresponding to the three different versions of HTML 4.01:

• XHTML 1.0 Strict is the XML equivalent to strict HTML 4.01, and includes elements and attributes that have not been marked deprecated in the HTML 4.01 specification. As of May 25 2011, XHTML 1.0 Strict is the document type used for the homepage of the website of the World Wide Web Consortium.
• XHTML 1.0 Transitional is the XML equivalent of HTML 4.01 Transitional, and includes the presentational elements (such as center, font and strike) excluded from the strict version.
• XHTML 1.0 Frameset is the XML equivalent of HTML 4.01 Frameset, and allows for the definition of frameset documents—a common Web feature in the late 1990s.

The second edition of XHTML 1.0 became a W3C Recommendation in August 2002.^[31]

[edit] Modularization of XHTML

Modularization provides an abstract collection of components through which XHTML can be subsetted and extended. The feature is intended to help XHTML extend its reach onto emerging platforms, such as mobile devices and Web-enabled televisions. The initial draft of Modularization of XHTML became available in April 1999, and reached Recommendation status in April 2001.^[32]
The first XHTML Family Markup Languages to be developed with this technique were XHTML 1.1 and XHTML Basic 1.0. Another example is XHTML-Print (W3C Recommendation, September 2006), a language designed for printing from mobile devices to low-cost printers.^[33]
In October 2008 Modularization of XHTML was superseded by XHTML Modularization 1.1, which adds an XML Schema implementation. It was itself superseded by a second edition in July 2010.^[34]

[edit] XHTML 1.1—Module-based XHTML

XHTML 1.1 evolved out of the work surrounding the initial Modularization of XHTML specification. The W3C released a first draft in September 1999; Recommendation status was reached in May 2001.^[35] The modules combined within XHTML 1.1 effectively recreate XHTML 1.0 Strict, with the addition of ruby annotation elements (ruby, rbc, rtc, rb, rt and rp) to better support East-Asian languages. Other changes include removal of the name attribute from the a and map elements, and (in the first edition of the language) removal of the lang attribute in favour of xml:lang.
Although XHTML 1.1 is largely compatible with XHTML 1.0 and HTML 4, in August 2002 the Working Group issued a formal Note advising that it should not be transmitted with the HTML media type.^[36] With limited browser support for the alternate application/xhtml+xml media type, XHTML 1.1 proved unable to gain widespread use. In January 2009 a second edition of the document (XHTML Media Types - Second Edition) was issued, relaxing this restriction and allowing XHTML 1.1 to be served as text/html.^[37]
A second edition of XHTML 1.1 was issued on 23 November 2010, which addresses various errata and adds an XML Schema implementation not included in the original specification.^[38] (It was first released briefly on 7 May 2009 as a "Proposed Edited Recommendation"^[39] before being rescinded on 19 May due to unresolved issues.)^[40]

[edit] XHTML Basic and XHTML-MP

To support constrained devices, XHTML Basic was created by the W3C; it reached Recommendation status in December 2000.^[41] XHTML Basic 1.0 is the most restrictive version of XHTML, providing a minimal set of features that even the most limited devices can be expected to support.
The Open Mobile Alliance and its predecessor the WAP Forum released three specifications between 2001 and 2006 that extended XHTML Basic 1.0. Known as XHTML Mobile Profile or XHTML-MP, they were strongly focused on uniting the differing markup languages used on mobile handsets at the time. All provide richer form controls than XHTML Basic 1.0, along with varying levels of scripting support.
XHTML Basic 1.1 became a W3C Recommendation in July 2008, superseding XHTML-MP 1.2.^[42] XHTML Basic 1.1 is almost but not quite a subset of regular XHTML 1.1. The most notable addition over XHTML 1.1 is the inputmode attribute—also found in XHTML-MP 1.2—which provides hints to help browsers improve form entry.
XHTML Basic 1.1 - Second Edition became a W3C Recommendation in November 2010.^[43]

[edit] XHTML 1.2

The XHTML 2 Working Group considered the creation of a new language based on XHTML 1.1.^[44] If XHTML 1.2 was created, it would include WAI-ARIA and role attributes to better support accessible web applications, and improved Semantic Web support through RDFa. The inputmode attribute from XHTML Basic 1.1, along with the target attribute (for specifying frame targets) might also be present. The XHTML2 WG had not been chartered to carry out the development of XHTML1.2. Since the W3C announced that it does not intend to recharter the XHTML2 WG,^[45] and closed the WG in December 2010, this means that XHTML 1.2 proposal would not eventuate.

[edit] XHTML 2.0

Between August 2002 and July 2006 the W3C released eight Working Drafts of XHTML 2.0, a new version of XHTML able to make a clean break from the past by discarding the requirement of backward compatibility. This lack of compatibility with XHTML 1.x and HTML 4 caused some early controversy in the web developer community.^[46] Some parts of the language (such as the role and RDFa attributes) were subsequently split out of the specification and worked on as separate modules, partially to help make the transition from XHTML 1.x to XHTML 2.0 smoother. A ninth draft of XHTML 2.0 was expected to appear in 2009, but on July 2, 2009, the W3C decided to let the XHTML2 Working Group charter expire by that year's end, effectively halting any further development of the draft into a standard.^[45] Instead, XHTML 2.0 and its related documents were released as W3C Notes.^[47][48]
New features introduced by XHTML 2.0 include:

• HTML forms will be replaced by XForms, an XML-based user input specification allowing forms to be displayed appropriately for different rendering devices.
• HTML frames will be replaced by XFrames.
• The DOM Events will be replaced by XML Events, which uses the XML Document Object Model.
• A new list element type, the nl element type, will be included to specifically designate a list as a navigation list. This will be useful in creating nested menus, which are currently created by a wide variety of means like nested unordered lists or nested definition lists.
• Any element will be able to act as a hyperlink, e. g., <li href="articles.html">Articles</li>, similar to XLink. However, XLink itself is not compatible with XHTML due to design differences.
• Any element will be able to reference alternative media with the src attribute, e. g., <p src="lbridge.jpg" type="image/jpeg">London Bridge</p> is the same as <object src="lbridge.jpg" type="image/jpeg"><p>London Bridge</p></object>.
• The alt attribute of the img element has been removed: alternative text will be given in the content of the img element, much like the object element, e. g., <img src="hms_audacious.jpg">HMS <span class="italic">Audacious</span></img>.
• A single heading element (h) will be added. The level of these headings are determined by the depth of the nesting. This allows the use of headings to be infinite, rather than limiting use to six levels deep.
• The remaining presentational elements i, b and tt, still allowed in XHTML 1.x (even Strict), will be absent from XHTML 2.0. The only somewhat presentational elements remaining will be sup and sub for superscript and subscript respectively, because they have significant non-presentational uses and are required by certain languages. All other tags are meant to be semantic instead (e. g. <strong> for strong or bolded text) while allowing the user agent to control the presentation of elements via CSS.
• The addition of RDF triple with the property and about attributes to facilitate the conversion from XHTML to RDF/XML.

[edit] XHTML5

Main article: HTML5

HTML5 initially grew independently of the W3C, through a loose group of browser manufacturers and other interested parties calling themselves the WHATWG, or Web Hypertext Application Technology Working Group. The WHATWG announced the existence of an open mailing list in June 2004, along with a website bearing the strapline “Maintaining and evolving HTML since 2004.”^[49] The key motive of the group was to create a platform for dynamic web applications; they considered XHTML 2.0 to be too document-centric, and not suitable for the creation of internet forum sites or online shops.^[50]
In April 2007, the Mozilla Foundation and Opera Software joined Apple in requesting that the newly rechartered HTML Working Group of the W3C adopt the work, under the name of HTML 5.^[51] The group resolved to do this the following month,^[52] and the First Public Working Draft of HTML5 was issued by the W3C in January 2008. The most recent W3C Working Draft was published in January 2011.^[28]
HTML5 has both a regular text/html serialization and an XML serialization, which is known as XHTML5. In addition to the markup language, the specification includes a number of application programming interfaces. The Document Object Model is extended with APIs for editing, drag-and-drop, data storage and network communication.
The language is more compatible with HTML 4 and XHTML 1.x than XHTML 2.0, due to the decision to keep the existing HTML form elements and events model. It adds many new elements not found in XHTML 1.x, however, such as section and aside.
The most recent draft includes WAI-ARIA support.^[28]

[edit] Semantic content in XHTML

XHTML+RDFa is an extended version of the XHTML markup language for supporting RDF through a collection of attributes and processing rules in the form of well-formed XML documents. This host language is one of the techniques used to develop Semantic Web content by embedding rich semantic markup.

[edit] Valid XHTML documents

An XHTML document that conforms to an XHTML specification is said to be valid. Validity assures consistency in document code, which in turn eases processing, but does not necessarily ensure consistent rendering by browsers. A document can be checked for validity with the W3C Markup Validation Service. In practice, many web development programs provide code validation based on the W3C standards.

[edit] Root element

The root element of an XHTML document must be html, and must contain an xmlns attribute to associate it with the XHTML namespace. The namespace URI for XHTML is http://www.w3.org/1999/xhtml. The example tag below additionally features an xml:lang attribute to identify the document with a natural language:

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">

[edit] DOCTYPEs

Main article: DOCTYPE

In order to validate an XHTML document, a Document Type Declaration, or DOCTYPE, may be used. A DOCTYPE declares to the browser the Document Type Definition (DTD) to which the document conforms. A Document Type Declaration should be placed before the root element.
The system identifier part of the DOCTYPE, which in these examples is the URL that begins with http://, need only point to a copy of the DTD to use, if the validator cannot locate one based on the public identifier (the other quoted string). It does not need to be the specific URL that is in these examples; in fact, authors are encouraged to use local copies of the DTD files when possible. The public identifier, however, must be character-for-character the same as in the examples.

[edit] XML declaration

A character encoding may be specified at the beginning of an XHTML document in the XML declaration when the document is served using the application/xhtml+xml MIME type. (If an XML document lacks encoding specification, an XML parser assumes that the encoding is UTF-8 or UTF-16, unless the encoding has already been determined by a higher protocol.)
For example:

<?xml version="1.0" encoding="UTF-8" ?>

The declaration may be optionally omitted because it declares as its encoding the default encoding. However, if the document instead makes use of XML 1.1 or another character encoding, a declaration is necessary. Internet Explorer prior to version 7 enters quirks mode, if it encounters an XML declaration in a document served as text/html.

[edit] Common errors

Some of the most common errors in the usage of XHTML are:

• Not closing empty elements (elements without closing tags in HTML4)
  • Incorrect: <br>
  • Correct: <br />
    Note that any of these is acceptable in XHTML: <br></br>, <br/>, and <br />. Older HTML-only browsers interpreting it as HTML will generally accept <br> and <br />.
• Not closing non-empty elements
  • Incorrect: <p>This is a paragraph.<p>This is another paragraph.
  • Correct: <p>This is a paragraph.</p><p>This is another paragraph.</p>
• Improperly nesting elements (Note that this would also be invalid in HTML)
  • Incorrect: <em><strong>This is some text.</em></strong>
  • Correct: <em><strong>This is some text.</strong></em>
• Not putting quotation marks around attribute values
  • Incorrect: <td rowspan=3>
  • Incorrect: <td rowspan='3">
  • Correct: <td rowspan="3">
  • Correct: <td rowspan='3'>
• Using the ampersand character outside of entities (Note that this would also be invalid in HTML)
  • Incorrect: <title>Cars & Trucks</title>
  • Correct: <title>Cars &amp; Trucks</title>
  • Incorrect: <a href="index.php?page=news&id=5">News</a>
  • Correct: <a href="index.php?page=news&amp;id=5">News</a>
• Failing to recognize that XHTML elements and attributes are case sensitive
  • Incorrect: <BODY><P ID="ONE">The Best Page Ever</P></BODY>
  • Correct: <body><p id="ONE">The Best Page Ever</p></body>
• Using attribute minimization
  • Incorrect: <textarea readonly>READ-ONLY</textarea>
  • Correct: <textarea readonly="readonly">READ-ONLY</textarea>
• Misusing CDATA, script-comments and xml-comments when embedding scripts and stylesheets.
  • This problem can be avoided altogether by putting all script and stylesheet information into separate files and referring to them as follows in the XHTML head element.

<link rel="stylesheet" href="/style/screen.css" type="text/css" />
 <script type="text/javascript" src="/script/site.js"></script>

Note: The format <script …></script>, rather than the more concise <script … />, is required for HTML compatibility when served as MIME type text/html.

• If an author chooses to include script or style data inline within an XHTML document, different approaches are recommended as shown in the examples below, depending whether the author intends to serve the page as application/xhtml+xml and target only fully conformant browsers, or serve the page as text/html and try to obtain usability in Internet Explorer 6 and other non-conformant browsers.

[edit] Backward compatibility

XHTML 1.x documents are mostly backward compatible with HTML 4 user agents when the appropriate guidelines are followed. XHTML 1.1 is essentially compatible, although the elements for ruby annotation are not part of the HTML 4 specification and thus generally ignored by HTML 4 browsers. Later XHTML 1.x modules such as those for the role attribute, RDFa and WAI-ARIA degrade gracefully in a similar manner.
XHTML 2.0 is significantly less compatible, although this can be mitigated to some degree through the use of scripting. (This can be simple one-liners, such as the use of “document.createElement()” to register a new HTML element within Internet Explorer, or complete JavaScript frameworks, such as the FormFaces implementation of XForms.)

[edit] Examples

The following are examples of XHTML 1.0 Strict, with both having the same visual output. The former one follows the HTML Compatibility Guidelines of the XHTML Media Types Note while the latter one breaks backward compatibility, but provides cleaner markup.^[37]

Media type recommendation for the examples:

Media type	Example 1	Example 2
application/xhtml+xml	SHOULD	SHOULD
application/xml	MAY	MAY
text/xml	MAY	MAY
text/html	MAY	SHOULD NOT

Example 1.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
 <title>XHTML 1.0 Strict Example</title>
 <script type="text/javascript">
 //<![CDATA[
 function loadpdf() {
    document.getElementById("pdf-object").src="http://www.w3.org/TR/xhtml1/xhtml1.pdf";
 }
 //]]>
 </script>
 </head>
 <body onload="loadpdf()">
 <p>This is an example of an
 <abbr title="Extensible HyperText Markup Language">XHTML</abbr> 1.0 Strict document.<br />
 <img id="validation-icon"
    src="http://www.w3.org/Icons/valid-xhtml10"
    alt="Valid XHTML 1.0 Strict" /><br />
 <object id="pdf-object"
    name="pdf-object"
    type="application/pdf"
    data="http://www.w3.org/TR/xhtml1/xhtml1.pdf"
    width="100%"
    height="500">
 </object>
 </p>
 </body>
</html>

Example 2.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
 <head>
 <title>XHTML 1.0 Strict Example</title>
 <script type="application/javascript">
 <![CDATA[
 function loadpdf() {
    document.getElementById("pdf-object").src="http://www.w3.org/TR/xhtml1/xhtml1.pdf";
 }
 ]]>
 </script>
 </head>
 <body onload="loadpdf()">
 <p>This is an example of an
 <abbr title="Extensible HyperText Markup Language">XHTML</abbr> 1.0 Strict document.<br/>
 <img id="validation-icon"
    src="http://www.w3.org/Icons/valid-xhtml10"
    alt="Valid XHTML 1.0 Strict"/><br />
 <object id="pdf-object"
    type="application/pdf"
    data="http://www.w3.org/TR/xhtml1/xhtml1.pdf"
    width="100%"
    height="500"/>
 </p>
 </body>
</html>