The concept of machine tags is the core premise of RDF. RDF is essentially the standardized way of describing relationships in a structured way (in XML). In fac

The concept of machine tags is the core premise of RDF. RDF is essentially the standardized way of describing relationships in a structured way (in XML). In fact, an early version of RSS was based on RDF (RSS 0.9 stood for "RDF site summary). One of the downsides is that it's pretty hard for "average" folks to produce these feeds. There's a steep learning curve for modeling the relationships. Getting other sites to agree on a format, use it, and maintain it without breaking compatibility was hard.