Sustainability of Digital Formats: Planning for Library of Congress Collections
Sustainability of Digital Formats: Planning for Library of Congress Collections |
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | Extensible Markup Language (XML) |
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| Description |
Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). XML documents fall into two broad categories: data-centric and document-centric. Data-centric documents are those where XML is used as a data transport. Examples include sales orders, patient records, directory entries, and metadata records. One significant use of data-centric XML is for manifests (lists) of digital content; another is for metadata embedded into digital content files. Document-centric documents are those in which XML is used for its SGML-like capabilities, reflecting the structure of particular classes of documents, such as books with chapters, user manuals, newsfeeds and articles incorporating explicit metadata in addition to the text. An XML document's markup structure can be defined by a schema language and validated against a definition in that language. The initial, and as of 2008, most widely used schema languages are the Document Type Definition (DTD) language and W3C XML Schema. Other schema languages exist, including RDF and RELAX-NG. |
| Production phase | Can be used as initial, middle, or final-state format. |
| Relationship to other formats | |
| Has subtype | XML_1_0, XML (Extensible Markup Language) 1.0 |
| Has subtype | XML_1_1, XML (Extensible Markup Language) 1.1 |
| Has subtype | XML_DTD, Document Type Definition |
| Has subtype | XML_SCHEMA, W3C XML Schema Language |
| May contain | CSS, Cascading Style Sheet (CSS) Markup. May embed CSS markup directly or invoke an external CSS file. |
| Used by | IMF_Package, Interoperable Master Format (IMF). Used for mandatory Asset Map and Packing List in IMF Package |
| Used by | APK, Android Package |
| Used by | IPA, iOS App Store Package |
| Used by | XAP, Silverlight Application Package |
| Has modified version | Other entities have introduced variant format versions using the .zip extension, not strictly compatible with any particular chronological version of ZIP_PK, but using its extension capabilities. See Notes below for a brief discussion of variants and compatibility. |
| Has extension | ADM, Audio Definition Model |
| Has extension | PEF, Portable Embosser Format |
| LC experience or existing holdings | Used by LC to represent metadata records (including MARC bibliographic and authority records, MODS, METS) for web-compatible interchange, in particular using the Open Archives Initiative Protocol for Metadata Harvesting and SRU (Search/Retrieval via URL). |
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| LC preference | The Library of Congress Recommended Formats Statement (RFS) lists XML as a Preferred format for Textual Works - Digital, with included or accessible DTD/schema, XSD/XSL presentation stylesheet(s), and explicitly stated character encoding. LC will express preferences based on specific DTDs, W3C XML Schema instances, or instance documents in other schema languages for defining XML-based formats. LC will prefer XML that represents the structure of documents rather than layout. The RFS also lists the XML format as an Acceptable format for Textual Works - Digital for XML-based document formats with presentation stylesheets. In addition to textual works in digital form, the Recommended Formats Statement lists XML as a Preferred format schema for Dataset metadata, for packaging data for Video - File-Based and Physical Media and for metadata for Audio Works - Media Independent (digital). The RFS also lists XML as an Acceptable format for metadata for photographs in digital form, other graphic images in digital form, and 2D and 3D Computer Aided Design vector images and scanned 3D objects (output from photogrammetry scanning). |
| Disclosure | Open standard. Developed by W3C (World Wide Web Consortium). To be useful for interoperability or long-term content preservation, an XML document must be associated with a schema specification for the elements and tags it contains. Such schema specifications (see XML_DTD and XML_XSD) must also be disclosed. |
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| Documentation | Maintained by W3C [http://www.w3.org/XML/]. Specifications for the two versions as of 2008 are at Extensible Markup Language (XML) 1.0 and Extensible Markup Language (XML) 1.1. |
| Adoption | Very widely adopted as the basis for interchange of documents and data over the Web. Many generic tools exist, including free and open source software. Major software vendors have all incorporated support for XML in some form. |
| Licensing and patents | None |
| Transparency | XML is human-readable and designed for straightforward automatic parsing. For the contents to be understood, a well-documented DTD, XML Schema, or other specification is needed. Human-comprehensible element tags are advantageous for transparency. |
| Self-documentation |
XML is widely used as a syntax for metadata, and metadata for all purposes can be embedded in XML documents with appropriate schema specifications. Accessibility Features XML-based formats have good support for accessibility features. According to W3C's XML Accessibility Guidelines, XML-based formats can include features that promote accessibility depending on implementation. This document outlines some techniques to achieve this, including the following:
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| External dependencies | None |
| Technical protection considerations | None |
| Text | |
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| Normal rendering | XML can represent all UNICODE characters, with UTF-8 being the default character encoding. XML tagging offers potential for explicitly representing logical structure of text, such as paragraphs and headings, and character emphasis (bold, italics, etc.). Effective support for normal rendering is dependent on an appropriate DTD or schema specification. |
| Integrity of document structure | XML is ideal for representing document structure. |
| Integrity of layout and display | For textual content, best practice is to have the XML represent the logical document structure and use stylesheets to render the text in a form appropriate for the end user. |
| Support for mathematics, formulae, etc. | Requires specialized markup (e.g., MathML) and corresponding rendering engine. Scholars in many scientific disciplines are not satisfied with the performance of such rendering engines. |
| Functionality beyond normal rendering | Depends on particular DTD or schema specification. |
| Tag | Value | Note |
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| Filename extension | xml |
Common practice for XML document instances is to use the .xml extension. The particular schema or DTD should be declared within the document. Some schemas specify the use of different file extensions. |
| Internet Media Type | text/xml application/xml |
If an XML document is readable by casual users, text/xml is preferred. See RFC 3023 for further details. |
| Magic numbers | See note. | Although no byte sequences can be counted on to always be present, XML MIME entities in ASCII-compatible charsets (including UTF-8) often begin with hexadecimal 3C 3F 78 6D 6C ("<?xml"), and those in UTF-16 often begin with hexadecimal FE FF 00 3C 00 3F 00 78 00 6D 00 6C or FF FE 3C 00 3F 00 78 00 6D 00 6C 00 (the Byte Order Mark (BOM) followed by "<?xml"). See RFC 3023 for further details. |
| Pronom PUID | fmt/101 |
See http://www.nationalarchives.gov.uk/PRONOM/fmt/101. |
| Wikidata Title ID | Q2115 |
See https://www.wikidata.org/wiki/Q2115. |
| Other | NF00654 |
See https://www.archives.gov/files/lod/dpframework/id/NF00654.ttl. |
| General | The original design goals for XML were:
Style sheets, for example in CSS or XSLT, can be associated with XML documents for presentation of XML files on the web. See Associating Style Sheets with XML documents, which includes an example with <?xml-stylesheet> processing instructions for associating external CSS style sheets with an XML file. The XHTML format specification also includes a <link> element that can be used to invoke external style sheets. |
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| History | "XML is primarily intended to meet the requirements of large-scale Web content providers for industry-specific markup, vendor-neutral data exchange, media-independent publishing, one-on-one marketing, workflow management in collaborative authoring environments, and the processing of Web documents by intelligent clients. It is also expected to find use in certain metadata applications. XML is fully internationalized for both European and Asian languages, with all conforming processors required to support the Unicode character set in both its UTF-8 and UTF-16 encodings. The language is designed for the quickest possible client-side processing consistent with its primary purpose as an electronic publishing and data interchange format." [from 1997-12-08 W3C press release] |
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