Example Catalog.xml
The following simple catalog shows how one might provide locally-cached DTDs for an XHTML page validation tool, for example.
This catalog makes it possible to resolve -//W3C//DTD XHTML 1.0 Strict//EN to the local URI dtd/xhtml1/xhtml1-strict.dtd. Similarly, it provides local URIs for two other public IDs.
Note that the document above includes a DOCTYPE - this may cause the parser to attempt to access the system ID URL for the DOCTYPE (i.e. http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd) before the catalog resolver is fully functioning, which is probably undesirable. To prevent this, simply remove the DOCTYPE declaration.
The following example shows this, and also shows the equivalent
Read more about this topic: XML Catalog