To sum up, the "photocasting" feature centers around a single
undocumented extension element in a namespace that doesn't need to be
declared. iPhoto 6 doesn't understand the first thing about HTTP, the
first thing about XML, or the first thing about RSS. It ignores
features of HTTP that Netscape 4 supported in 1996, and mis-implements
features of XML that Microsoft got right in 1997. It ignores 95% of
RSS and Atom and gets most of the remaining 5% wrong.

Link via Tim Bray's Ongoing.