Working off-network

Not always connected If you're working on a laptop, and the application you're working with uses a validating parser... you may be waiting a while. Even if you're connected, network failures (like the 30,000 other users who want to retrieve the DTD you need) may cause inefficiency and failure.
The loss of public identifiers Although public identifiers are included in the XML specification, the lack of infrastructure for these SGML creations has led to little support for them. The W3C does use them in XHTML 1.1 modularization, providing SGML Open Catalog Entries that can be used to enable PUBLIC identifier resolution and caching. Arbortext recently released tools for adding such support to SAX-based parsers.
The need for intelligent caching If these resources remain stable, some kind of caching seems like a good solution, possibly supported by a public identifier infrastructure. However, it still leaves the uncertainty of what to do when a document is opened for the first time. These kinds of problems will continue to grow as network infrastructure become more important, and may be unavoidable.