OPML (Outline Processor Markup Language) is an

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

format for outlines (defined as "a tree, where each node contains a set of named attributes with string values"). Originally developed by UserLand as a native file format for the outliner application in its Radio UserLand product, it has since been adopted for other uses, the most common being to exchange lists of web feeds between web feed aggregators. 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 di ...

feed lists in OPML format is available in

Mozilla Thunderbird Mozilla Thunderbird is a free and open-source cross-platform email client, personal information manager, news client, RSS and chat client developed by the Mozilla Foundation and operated by subsidiary MZLA Technologies Corporation. The projec ...

, and many 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. 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. # 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 (see XML and MIME).

Validation

Dave Winer 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 * XOXO – Extensible Open XHTML Outlines * XBEL – XML Bookmark Exchange Language

References

External links