RSS 1.0 Validator is a Scripts software developed by XMLPitstop. After our trial and test, the software is proved to be official, secure and free. Here is the official description for RSS 1.0 Validator:

BS Editor: DTD based validation does not provide the kind of flexibility required in many applications. A DTD is limited in the structures that it can specify, and cannot validate element content, e.g. to determine they have the correct length or format.