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A microformat (sometimes abbreviated μF or uF) is a web-based^[1] data formatting approach that seeks to re-use existing content as metadata, using only XHTML and HTML classes^[2] and attributes.^[3] This approach is intended to allow information intended for end-users (such as contact information, geographic coordinates, calendar events, and the like) to also be automatically processed by software.

Although the content of web pages is technically already capable of "automated processing," and has been since the inception of the web, there are certain limitations. This is because the traditional markup tags used to display information on the web do not describe what the information means.^[4] Microformats are intended to bridge this gap by attaching semantics, and thereby obviate other, more complicated methods of automated processing, such as natural language processing or screen scraping. The use, adoption and processing of microformats enables data items to be indexed, searched for, saved or cross-referenced, so that information can be reused or combined.^[4]

Current microformats allow the encoding and extraction of events, contact information, social relationships and so on. More are being developed. Version 3 of the Firefox browser,^[5] as well as version 8 of Internet Explorer^[6] are expected to include native support for microformats.

Microformats emerged as part of a grassroots movement^[4] to make recognizable data items (such as events, contact details or geographical locations) capable of automated processing by software, as well as directly readable by end-users.^[4][7] Link-based microformats emerged first. These include vote links that express opinions of the linked page, which can be tallied into instant polls by search engines.^[8]

As the microformats community grew, CommerceNet, a nonprofit organization that promotes electronic commerce on the Internet, helped sponsor and promote the technology and support the microformats community in various ways.^[8] CommerceNet also helped co-found the microformats community site microformats.org.^[8]

Neither CommerceNet nor Microformats.org is a standards body. The microformats community is an open wiki, mailing list, and Internet relay chat (IRC) channel.^[8] Most of the existing microformats were created at the Microformats.org wiki and associated mailing list, by a process of gathering examples of web publishing behaviour, then codifying it. Some other microformats (such as rel=nofollow and unAPI) have been proposed, or developed, elsewhere.

XHTML and HTML standards allow for semantics to be embedded and encoded within the attributes of markup tags. Microformats take advantage of these standards by indicating the presence of metadata using the following attributes:

• class
• rel
• rev (in one case, otherwise deprecated in microformats^[3])

For example, in the text "The birds roosted at 52.48,-1.89" is a pair of numbers which may be understood, from their context, to be a set of geographic coordinates. By wrapping them in spans (or other HTML elements) with specific class names (in this case geo, latitude and longitude, all part of the geo microformat specification):

The birds roosted at
    class="geo">
      class="latitude">52.48>,
      class="longitude">-1.89>
   >

machines can be told exactly what each value represents and can then perform a variety of tasks such as indexing it, looking it up on a map and exporting it to a GPS device.

In this example, the contact information is presented as follows:

>
   >Joe Doe>
   >The Example Company>
   >604-555-1234>
    href="http://example.com/">http://example.com/>
 >

With hCard microformat markup, that becomes:

 class="vcard">
    class="fn">Joe Doe>
    class="org">The Example Company>
    class="tel">604-555-1234>
    class="url" href="http://example.com/">http://example.com/>
 >

Here, the formatted name (fn), organisation (org), telephone number (tel) and web address (url) have been identified using specific class names and the whole thing is wrapped in class="vcard", which indicates that the other classes form an hCard (short for "HTML vCard)") and are not merely coincidentally named. Other, optional, hCard classes also exist. It is now possible for software, such as browser plug-ins, to extract the information, and transfer it to other applications, such as an address book.

Several microformats have been developed to enable semantic markup of particular types of information.

• hAtom - for marking up Atom (standard) feeds from within standard HTML
• hCalendar - for events
• hCard - for contact information; includes:

• adr (microformat) - for postal addresses
• geo (microformat) - for geographical coordinates (latitude, longitude)

• hReview - for reviews
• hResume - for resumes or CVs
• rel-directory (file systems) - for distributed directory creation and inclusion
• rel-nofollow, an attempt to discourage 3rd party content spam (e.g. spam in blogs).
• rel-tag (metadata) - for decentralized tagging (Folksonomy)
• xFolk - for tagged links
• XHTML Friends Network (XFN) - for social relationships
• XOXO - for lists and outlines

Among the many proposed microformats,^[9] the following are undergoing active development:

• citation - for citing references
• currency - for amounts of money
• geo extensions - for places on Mars, The Moon, and other such bodies; for altitude; and for collections of waypoints marking routes or boundaries
• species - For the names of living things.
• measurements - For physical quantities, structured data-values.

Using microformats within HTML code provides additional formatting and semantic data that can be used by applications. These could be applications that collect data about on-line resources, such as web crawlers, or desktop applications such as e-mail clients or scheduling software.

Several browser extensions, such as Operator, provide the ability to detect microformats within an HTML document and export them into formats compatible with contact management and calendar utilities, such as Microsoft Outlook.

Microsoft expressed a desire to incorporate Microformats into upcoming projects;^[10] as have other software companies.

In Wikipedia - and more generally in MediaWiki - microformats are used as part of templates like {{coord}}.

Various commentators have offered review and discussion on the design principles and practical aspects of microformats. Additionally, microformats have been compared to other approaches that seek to serve the same or similar purpose.^[11] From time to time, there is criticism of a single, or all, microformats.^[11] Documented efforts to advocate both the spread and use of microformats are known to exist as well.^{[12] [13]} Håkon Wium Lie has said we will see "a bunch of microformats being developed, and that’s how the semantic web will be built." ^[14]

Rohit Khare has stated that reduce, reuse, and recycle is "shorthand for several design principles" that motivated the development and practices behind microformats.^[8][15] These aspects can be summarized as follows:

• Reduce: favor the simplest solutions and focus attention on specific problems;
• Reuse: work from experience and favor examples of current practice;
• Recycle: encourage modularity and the ability to embed, valid XHTML can be reused in blog posts, RSS feeds, and anywhere else you can access the web.^[8]

Because some microformats make use of title attribute of HTML's abbr element to conceal machine-readable data (particularly date-times and geographical coordinates) in the "abbr design pattern", the plain text content of the element is inaccessible to those screenreaders that expand abbreviations.^[16]

As part of the open participation model of the microformats community, some claim a hard-and-fast rule for participation has been that any specifications resulting from community participation be free from restrictions on use based on intellectual property. This means that microformats must not be limited by copyrights licenses or patents, and must be royalty-free. It is not clear what enforceability, if any, the 'no intellectual property' rule has.

Microformats are not the only solution for providing "more intelligent data" on the web. Alternative approaches exist and are under development as well. For example, the use of XML markup and standards of the semantic web are cited as alternative approaches. ^[8] Some contrast these with microformats in that they do not necessarily coincide with the design principles of "reduce, reuse, and recycle", at least not to the same extent.^[8]

One advocate of microformats, Tantek Çelik, characterized a problem with alternative approaches:

For some applications the use of other approaches may be valid. If one wishes to use microformat-style embedding but the type of data one wishes to embed does not map to an existing microformat, one can use RDFa to embed arbitrary vocabularies into HTML. An example of this would be embedding domain-specific scientific data on the Web like zoological or chemical data, where no microformat for such data exists. Furthermore, standards such as W3C's GRDDL allow microformats to be converted into data compatible with the Semantic Web.^[17]

Another advocate of microformats, Ryan King, put the compatibility of microformats with other approaches this way:

• COinS
• GRDDL
• Intelligent agents
• RDFa
• S5 file format
• Simple HTML Ontology Extensions
• Web crawlers
• Tag (metadata)

• Suda, Brian (2006). Using Microformats. O'Reilly Media. ISBN 978-0-596-52817-1. 

• microformats.org
• Microformats Primer
• A four-part discussion of Microformats, UI issues, and possible presentation in Firefox 3 by Alex Faaborg of Mozilla
• "hAccessibility" - Web Standards Project discussion of inaccessible microformat design pattern