Sustainability of Digital Formats: Planning for Library of Congress Collections

Outline Processor Markup Language (OPML) is an XML-based format, specifically XML 1.0, for outlines where each node contains a set of named attributes with string values. An outline, in this context, is "a tree, where each node contains a set of named attributes with string values." The OPML specification, which is open, is developed and maintained by Dave Winer at opml.org. The current version, covered in this document, is version 2.0. See History for information about previous versions.

Structure of OPML

OPML is a hierarchical, ordered list of elements:

• opml: root element that must contain the version attribute and one head and one body element. For OPML 2.0, the value is opml version="2.0". See File Signifiers section for values for other OPML versions.
• head: contains zero or more optional elements: title (title of document); dateCreated (date-time when the document was created); dateModified (date-time when the document was last modified); ownerName (string, the owner of the document); ownerEmail (string, email address of the owner of the document); ownerId (http address of a web page that contains information that allows a human reader to communicate with the author of the document via email or other means. It also may be used to identify the author. No two authors have the same ownerId); docs (http address of documentation for the format used in the OPML file); expansionState (comma-separated list of line numbers that are expanded); vertScrollState (number, stating which line of the outline is displayed on the top line of the window): windowTop (number, the pixel location of the top edge of the window); windowLeft (number, the pixel location of the left edge of the window); windowBottom (number, the pixel location of the bottom edge of the window); windowRight (number, the pixel location of the right edge of the window)
• body: contains one or more outline elements.
• outline: element containing at least one required attribute, text, and zero or more additional attributes and zero or more sub-elements. No attribute may be repeated within the same outline element. Other optional attributes include type, isComment, isBreakpoint, created, and category.

Subscription lists, especially RSS or Really Simple Syndication feeds, are identified as sub-element of the body element with a node of type rss or an outline element that contains nodes of type rss.

Uses of OPML

According to the specification, the goals of OPML are "to provide a way to exchange information between outliners and Internet services that can be browsed or controlled through an outliner"; serve as a "file format for an outliner application, which is why OPML files may contain information about the size, position and expansion state of the window the outline is displayed in"; and, as a "format for exchanging subscription lists between feed readers and aggregators" especially RSS feeds. opml.org includes sample files for various uses.

Issues with OPML

Wikidata lists some criticisms of OPML including issues with the requirement that syntax in date-time elements must conform to the outdated RFC 822 specification which has been long superseded by RFC 5322, issues with the "expansionState" attribute, and general potential interoperability issues because "OPML documents relies almost entirely on the undocumented conventions of content producers." Issues with the OPML specification are included on the opml.org GitHub repo.

According to Renée Christian in What are OPML Files, and How Do You Open, Edit, and Use Them?, "OPML was developed by a US-based software company called UserLand in 2000. UserLand used OPML as a native file format for Radio UserLand, an RSS aggregator and blogging tool." Dave Winer, the designer of the OPML format , founded UserLand in 1988. The current version of OPML is 2.0. The first version, version 1.0, was published in 2000. Version 1.1 was never formally finalized or published although a version 1.1 sample file is available and displays the version number in the header metadata.