This service validates your document against the RSS 1.1 Specification to let you know whether or not it is conformant. A conformant feed is more likely to be able to be parsed correctly by aggregators and other user agents. Examples from the RSS 1.1 test suite:
You may also like to browse the python source code for this validator; it requires the RSS 1.1 validator.py module. Related resources: RSS Feed Converter, A Rough Guide to RSS 1.1.