OPML (Outline Processor Markup Language) is an

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing data. It defines a set of rules for encoding electronic document, documents in a format that is both human-readable and Machine-r ...

format for

outlines Outline or outlining may refer to: * Outline (list), a document summary, in hierarchical list format * Code folding, a method of hiding or collapsing code or text to see content in outline form * Outline drawing, a sketch depicting the outer edge ...

(defined as "a tree, where each node contains a set of named attributes with string values"). Originally developed by

UserLand Software UserLand Software is a US-based software company, founded in 1988, that sells web content management, as well as blogging software packages and services. Company history Dave Winer founded the company in 1988 after leaving Symantec in the spr ...

as a native file format for the

outliner An outliner (or outline processor) is a specialized type of text editor (word processor) used to create and edit Outline (list), outlines, which are text files which have a tree structure or a tree view, for organization. Textual information is co ...

application in its Radio UserLand product, it has since been adopted for other uses, the most common being to exchange lists of

web feeds On the World Wide Web, a web feed (or news feed) is a data format used for providing users with frequently updated content. Content distributors ''syndicate'' a web feed, thereby allowing users to ''subscribe'' a channel to it by adding the fee ...

between web

feed aggregator Feed or The Feed may refer to: Animal foodstuffs * Animal feed, food given to domestic animals in the course of animal husbandry ** Fodder, foodstuffs manufactured for animal consumption ** Forage, foodstuffs that animals gather themselves, su ...

s. The OPML specification defines an outline as a hierarchical, ordered list of arbitrary elements. The specification is fairly open which makes it suitable for many types of list data. Support for importing and exporting

RSS RSS ( RDF Site Summary or Really Simple Syndication) is a web feed that allows users and applications to access updates to websites in a standardized, computer-readable format. Subscribing to RSS feeds can allow a user to keep track of many ...

feed lists in OPML format is available in

Mozilla Thunderbird Mozilla Thunderbird is a free and open-source email client that also functions as a personal information manager with a Digital calendar, calendar and contactbook, as well as an RSS feed reader, chat client (IRC/XMPP/Matrix (protocol), Matrix), ...

and in most other RSS reader web sites and applications.

XML format

The XML elements in an OPML document are: ; <opml version="1.0"> : This is the root element. It must contain the version attribute and one ''head'' and one ''body'' element. ; <head> : Contains

metadata Metadata (or metainformation) is "data that provides information about other data", but not the content of the data itself, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive ...

. May include any of these optional elements: ''title'', ''dateCreated'', ''dateModified'', ''ownerName'', ''ownerEmail'', ''expansionState'', '' vertScrollState'', ''windowTop'', ''windowLeft'', ''windowBottom'', ''windowRight''. Each element is a simple text element. ''dateCreated'' and ''dateModified'' contents conform to the date-time format specified in . ''expansionState'' contains a comma-separated list of line numbers that should be expanded on display. The ''windowXXX'' elements define the position and size of the display window. An OPML processor may ignore all the ''head'' sub-elements. If the outline is opened inside another outline then the processor must ignore the window elements. ; <body> : Contains the content of the outline. Must have at least one ''outline'' element. ; <outline> : Represents a line in the outline. May contain any number of arbitrary attributes. Common attributes include ''text'' and ''type''. The outline element may contain any number of ''outline'' sub-elements.

Criticism

# The RFC 822 date format is considered obsolete, and amongst other things permits the representation of years as two digits. (RFC 822 has been superseded by RFC 2822 then RFC 5322.) In general, date and time formats should be represented according to

RFC 3339 ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the International Organization for Standardization (ISO) and was first published in 1988, with updates in ...

. # Due to the semantics of the "expansionState" attribute: #* The expansion state of the children of collapsed nodes cannot be stored #* When an outline element is expanded, collapsed, added, deleted or moved, the expansion state of all elements below it must be recalculated # Due to the arbitrary nature of the "type" attribute, and the acceptance of arbitrary attributes on "outline" elements, interoperability of OPML documents relies almost entirely on the undocumented conventions of content producers. # MIME types aren't clear; text/xml usually merely identifies the document as having XML content, while text/x-opml does not specify that the document is XML since it lacks the +xml suffix.

Validation

Dave Winer Dave Winer (born May 2, 1955, in Queens, New York City) is an American software developer, entrepreneur, and writer who resides in New York City. Winer is noted for his contributions to outliners, scripting, content management, and web service ...

, the founder and CEO of Userland, has made a request for comments on
proposal for validating OPML
and released a bet
OPML validator
Katy Ginger from DLESE has publishe

for validating OPML 2.0. There is also
DTD available for OPML 1.0

Alternative specifications

Alternative specifications have been proposed, notably these include OML, XOXO and XBEL. * OML – Outline Markup Language * XOXO – Extensible Open XHTML Outlines * XBEL – XML Bookmark Exchange Language

References

External links