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xFolk (RC1)

xFolk is a simple and open format for publishing collections of bookmarks. It better enables services for improving user experience and sharing data in web-based bookmarking software. xFolk may be embedded in (X)HTML, Atom, RSS, and arbitrary XML. It is one of several open microformat standards.


Draft Specification


Bud Gibson, The Community Engine


This specification is (C) 2005-2020 by the authors. However, the authors intend to submit (or already have submitted, see details in the spec) this specification to a standards body with a liberal copyright/licensing policy such as the GMPG, IETF, and/or W3C. Anyone wishing to contribute should read their copyright principles, policies and licenses (e.g. the GMPG Principles) and agree to them, including licensing of all contributions under all required licenses (e.g. CC-by 1.0 and later), before contributing.


This specification is subject to a royalty free patent policy, e.g. per the W3C Patent Policy, and IETF RFC3667 & RFC3668.

Inspiration and Acknowledgments

Thanks to: David Plaut who introduced me to the idea that items may partially belong to many categories at once. More recently, thanks to: Tantek Çelik, Kevin Marks, Steve Mallet, Brian DelVecchio, and François Hodierne who have contributed to the development of xFolk with thoughtful critiques and implementations.


Social bookmarking services let users save and tag bookmarks to share with other users. Over the past year, the number of these services has mushroomed to over 20, with popular examples including: del.icio.us, furl, de.lirio.us, jots, and blogmarks.

Lack of an open, interoperable data standard is a major issue in using social bookmarking services. An open standard would make it possible to easily collect social bookmark data and remix it to invent new services along the lines of gataga's experimental bookmark search engine. An open standard would also make it possible to write javascripts that work across services as some currently do for del.icio.us, enabling across-the-board improvements in user experience.

xFolk is an open social bookmarking standard that aims to achieve both benefits. Social bookmarking services that adopt xFolk will continue to differentiate themselves with data services. But, the data itself will be easy to manipulate by third party scripts and easy to share. As a result, bookmarking services will be able to offer a wider array of options to their users at low cost and focus on what truly differentiates them. Individuals or groups operating standalone bookmark repositories will have access to a wider array of functionality than they can produce on their own.

Semantic XHTML Design Principles

Note: the Semantic XHTML Design Principles were written primarily within the context of developing hCard and hCalendar, thus it may be easier to understand these principles in the context of the hCard design methodology (i.e. read that first). Tantek

XHTML is built on XML, and thus XHTML based formats can be used not only for convenient display presentation, but also for general purpose data exchange. In many ways, XHTML based formats exemplify the best of both HTML and XML worlds. However, when building XHTML based formats, it helps to have a guiding set of principles.

  1. Reuse the schema (names, objects, properties, values, types, hierarchies, constraints) as much as possible from pre-existing, established, well-supported standards by reference. Avoid restating constraints expressed in the source standard. Informative mentions are ok.
    1. For types with multiple components, use nested elements with class names equivalent to the names of the components.
    2. Plural components are made singular, and thus multiple nested elements are used to represent multiple text values that are comma-delimited.
  2. Use the most accurately precise semantic XHTML building block for each object etc.
  3. Otherwise use a generic structural element (e.g. <span> or <div>), or the appropriate contextual element (e.g. an <li> inside a <ul> or <ol>).
  4. Use class names based on names from the original schema, unless the semantic XHTML building block precisely represents that part of the original schema. If names in the source schema are case-insensitive, then use an all lowercase equivalent. Components names implicit in prose (rather than explicit in the defined schema) should also use lowercase equivalents for ease of use. Spaces in component names become dash '-' characters.
  5. Finally, if the format of the data according to the original schema is too long and/or not human-friendly, use <abbr> instead of a generic structural element, and place the literal data into the 'title' attribute (where abbr expansions go), and the more brief and human readable equivalent into the element itself. Further informative explanation of this use of <abbr>: Human vs. ISO8601 dates problem solved


Extensive analysis of social bookmarking services and linkblogs, reported on elsewhere, reveals that the data published by both bookmarking services and many different linkblogs are described by the following implied schema:

As demonstrated in early iterations of the xFolk standard, this general schema can be translated into markup by:

  1. creating a container element for each entry of class xfolkentry,
  2. using an <a> element for the bookmarked or tagged link of class taggedlink,
  3. using title attribute for the tagged link <a> element, if it exists, as the entry title, otherwise using the element value,
  4. using RelTag for the tags, and
  5. using a container element of class description, such as
xfolk-pt-br was last modified: Wednesday, December 31st, 1969