w3c-markup-validator 1.1-2 does not install on natty - Depends: w3c-sgml-lib but it is not installable

[Chris Bainbridge]

# apt-get install w3c-markup-validator
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
 w3c-markup-validator : Depends: w3c-sgml-lib but it is not installable
E: Broken packages

# apt-get install w3c-sgml-lib
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package w3c-sgml-lib is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'w3c-sgml-lib' has no installation candidate