OPML
OPML (Outline Processor Markup Language) is an XML 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 feed lists in OPML format is available in Mozilla Thunderbird, and many other RSS reader web sites and applications. XML format. The XML elements in an OPML document are: Validation. Dave Winer has made a request for comments on a proposal for validating OPML and released a beta OPML validator. Katy Ginger from DLESE has published an XML Schema Document for validating OPML 2.0. There is also a DTD available for OPML 1.0. Alternative specifications. Alternative specifications have been proposed, notably these include OML, XOXO and XBEL.