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==Variants==
As noted above, there are several different versions of RSS, falling into two major branches (RDF and 2.*).  The RDF, or RSS 1.* branch includes the following versions:
*RSS 0.90 was the original Netscape RSS version. This RSS was called ''[[Resource Description Framework|RDF]] Site Summary'', but was based on an early working draft of the RDF standard, and was not compatible with the final RDF Recommendation.
*RSS 1.0 is an open format by the [[RSS-DEV Working Group]], again standing for ''RDF Site Summary''. RSS 1.0 is an RDF format like RSS 0.90, but not fully compatible with it, since 1.0 is based on the final RDF 1.0 Recommendation.
*RSS 1.1 is also an open format and is intended to update and replace RSS 1.0. The specification is an independent draft not supported or endorsed in any way by the RSS-Dev Working Group or any other organization.

The RSS 2.* branch (initially UserLand, now Harvard) includes the following versions:
*RSS 0.91 is the simplified RSS version released by Netscape, and also the version number of the simplified version originally championed by [[Dave Winer]] from Userland Software. The Netscape version was now called ''Rich Site Summary''; this was no longer an RDF format, but was relatively easy to use.
*RSS 0.92 through 0.94 are expansions of the RSS 0.91 format, which are mostly compatible with each other and with Winer's version of RSS 0.91, but are not compatible with RSS 0.90. In all Userland RSS 0.9x specifications, RSS was no longer an acronym.
*RSS 2.0.1 has the internal version number 2.0. RSS 2.0.1 was proclaimed to be "frozen", but still updated shortly after release without changing the version number.  RSS now stood for ''Really Simple Syndication''.  The major change in this version is an explicit extension mechanism using XML namespaces.<ref name="W3C REC XML Namespace" >{{cite web
  |title=Namespaces in XML 1.0
  |url=http://www.w3.org/TR/REC-xml-names/
  |publisher=W3C
  |date=16 August 2006
  |edition=2nd
}}</ref>

For the most part, later versions in each branch are backward-compatible with earlier versions (aside from non-conformant RDF syntax in 0.90), and both versions include properly documented extension mechanisms using XML Namespaces, either directly (in the 2.* branch) or through RDF (in the 1.* branch).  Most syndication software supports both branches. [http://diveintomark.org/archives/2004/02/04/incompatible-rss "The Myth of RSS Compatibility"], an article written in 2004 by RSS critic and [[Atom (standard)|Atom]] advocate [[Mark Pilgrim]], discusses RSS version compatibility issues in more detail.

The extension mechanisms make it possible for each branch to track innovations in the other.  For example, the RSS 2.* branch was the first to support [[RSS Enclosures|enclosures]], making it the current leading choice for [[podcast]]ing, and {{as of|2005|lc=on}} is the format supported for that use by [[iTunes]] and other [[podcast]]ing software; however, an enclosure extension is now available for the RSS 1.* branch, [http://www.xs4all.nl/~foz/mod_enclosure.html mod_enclosure].  Likewise, the RSS 2.* core specification does not support providing full-text in addition to a synopsis, but the RSS 1.* markup can be (and often is) used as an extension.  There are also several common outside extension packages available, including a new proposal from [[Microsoft]] for use in [[Internet Explorer]] 7.

The most serious compatibility problem is with HTML markup. Userland's RSS reader&mdash;generally considered as the reference implementation&mdash;did not originally filter out [[HTML]] markup from feeds. As a result, publishers began placing HTML markup into the titles and descriptions of items in their RSS feeds. This behavior has become expected of readers, to the point of becoming a [[de facto]] standard, though there is still some inconsistency in how software handles this markup, particularly in titles. The RSS 2.0 specification was later updated to include examples of entity-encoded HTML; however, all prior plain text usages remain valid.

{{As of|2007|1}}, tracking data from www.syndic8.com indicates that the three main versions of RSS in current use are 0.91, 1.0, and 2.0. Of these, RSS 0.91 accounts for 13 percent of worldwide RSS usage and RSS 2.0 for 67 percent, while RSS 1.0 has a 17 percent share.<ref>[http://www.peachpit.com/articles/article.aspx?p=674690 Peachpit article]</ref> These figures, however, do not include usage of the rival web feed format [[Atom (standard)|Atom]]. {{As of|2008|8}}, the syndic8.com website is indexing 546,069 total feeds, of which 86,496 were some dialect of Atom and 438,102 were some dialect of RSS.<ref>[http://www.syndic8.com/stats.php?Section=feeds#tabtable Syndic8 stats table]</ref>

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