I'm not sure the situation with java.sun.com, but those provided by w3c do have a 90 days expiration (according to one of the links I've posted). In all cases,

I'm not sure the situation with java.sun.com, but those provided by w3c do have a 90 days expiration (according to one of the links I've posted). In all cases, since the DTDs are more or less versioned through their filenames, with quite a minimal rate of changes, caching them (even if not outright saving them forever) should be the default action.