5. Atom 0.3
Note that with the release of Atom 1.0 during Summer 2005, Atom 0.3 is deprecated. The above web page says the Atom 0.3 specification is made available for historical purposes only. It continues by saying DO NOT implement it or ship products conforming to it.
The Atom 0.3 specification says that content elements MAY have a "type" attribute, whose value indicates the media type of the content and if this is not present, its value MUST be considered to be "text/plain". The specification also says that content elements MAY have a "mode" attribute, whose value indicates the method used to encode the content. This can be one of the values "xml", "escaped" and "base64". If [it is] not present, its value MUST be considered to be "xml".
(a) either put HTML inside a CDATA section, as shown at http://bluebillinc.com/feed/atom/
(b) or they provide encoded HTML writing <p> as & lt;p& gt; as shown at http://ramble.oucs.ox.ac.uk/blog/stuart/atom.xml.