Outline Processor Markup Language (OPML)

Outline Processor Markup Language has evolved into a format used for exchanging subscription lists between RSS aggregators and RSS feed readers. Users can track their own RSS feeds as well as observe who is subscribing, where they are from and the feeds they have chosen.

OPML also has some shortcomings:

• The date format only allows two-digit years and the format does not conform to RFC 3339.
• The expansion state of some windows cannot be stored.
• When a window is altered or deleted, the windows below it must be recalculated.
• The arbitrary nature of the type attribute, and the use of arbitrary attributes on outline elements causes the interoperability of the documents produced to be almost completely dependent on the conventions of the content producers, which may be neither standard nor documented.
• There are problems with identifying created documents as XML format.