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Canonical XML - Wikipedia

From Wikipedia, the free encyclopedia

Canonical XML is a normal form of XML, intended to allow relatively simple comparison of pairs of XML documents for equivalence; for this purpose, the Canonical XML transformation removes non-meaningful differences between the documents. Any XML document can be converted to Canonical XML.

For example, XML permits whitespace to occur at various points within start-tags, and attributes to be specified in any order. Such differences are seldom if ever used to convey meaning, and so these forms are generally considered equivalent:

   <p class="a" secure="1">

   <p     secure   = "1"
             class='a'   >

In converting an arbitrary XML document to Canonical XML, attributes are encoded in a normative order (alphabetical by name), and with normative spacing and quoting (though with all namespace declarations placed ahead of regular attributes, and namespaced attributes sorted by namespace rather than prefix or qualified name). Thus, the second form above would be converted to the first.

Canonical XML specifies a number of other details, some of which are:

the UTF-8 encoding is used
line-ends are represented using the newline character 0x0A
whitespace in attribute values is normalized
entity references and non-special character references are expanded
CDATA sections are replaced with their character content
empty elements are encoded as start/end pairs, not using the special empty-element syntax
default attributes are made explicit
superfluous namespace declarations are deleted

According to the W3C, if two XML documents have the same canonical form, then the two documents are logically equivalent within the given application context (except for limitations regarding a few unusual cases).

However, in a special context users might care about special semantics beyond the generic logical equivalence with which Canonical XML is associated. For example, a steganography system could conceal information in an XML document by varying whitespace, attribute quoting and order, the use of hexadecimal vs. decimal numeric character references, and so on. Obviously converting such a file to Canonical XML would lose those specialized semantics. On the other hand, XML files that differ in their use of upper- vs. lower-case, or that use archaic versus modern spelling, and so on, might be considered equivalent for certain purposes. Such contexts are beyond the scope of Canonical XML.

External links

World Wide Web Consortium (W3C)

Products and
standards

Recommendations	ActivityPub Activity Streams ARIA Canonical XML CDF CSS Animations Flexbox Grid DOM EXI EmotionML Geolocation API HTML HTML5 IndexedDB ITS JSON-LD Linked Data Notifications MathML Micropub OWL PLS RDF Schema SISR SKOS SMIL SOAP SRGS SRI SSML SVG Filter Effects SCXML SHACL SPARQL Timed text VoiceXML WebAssembly WoT TD Web storage WSDL Webmention WebSub WebVTT WOFF XHTML +RDFa XML Base Encryption Events Information Set Namespace Schema Signature XForms XInclude XLink XOP XPath 2.0 3.x XPointer XProc XQuery XSL XSL-FO XSLT elements
Notes	IndieAuth XAdES XBL XHTML+SMIL XUP
Working drafts	CCXML CURIE EME InkML MSE RIF SMIL Timesheets sXBL WebGPU WebXR XFDL XFrames XMLHttpRequest
Guidelines	Web Content Accessibility Guidelines
Initiative	Markup Validation Service Web Accessibility Initiative Web Components
Deprecated	C-HTML HDML JSSS PGML VML WebPlatform
Obsoleted	P3P XHTML+MathML+SVG

Organizations

World Wide Web Foundation WHATWG
Elected groups	AB Board TAG
Working groups	CSS SVG WebAssembly WebAuthn
Community & business groups	Web Advertising BG WebAssembly CG
Closed groups	Device Description (DDWG) HTML Multimodal Interaction Activity (MMI)

Software

CERN httpd Libwww
Browsers	Line Mode (1990–) Arena (1993–98) Agora (1994–97) Argo (1994–97) Amaya (browser/editor, 1996–2012)

Conferences

International World Wide Web Conference (IW3C)
- Steering Committee (IW3C2)
- First conference ("WWW1", 1994)

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