w3c-markup-validator depends on w3c-sgml-lib (nonexistent in Natty)

On Natty, w3c-markup-validator (package version 1.1-2) is uninstallable, since it depends on the package w3c-sgml-lib, which exists only in Oneiric, and not in Natty. (I discovered this while investigating https://answers.launchpad.net/ubuntu/+question/163021; my testing was on a fully updated Natty amd64 system.)

ek@Apok:~$ apt-get -s install w3c-markup-validator
NOTE: This is only a simulation!
      apt-get needs root privileges for real execution.
      Keep also in mind that locking is deactivated,
      so don't depend on the relevance to the real current situation!
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

ek@Apok:~$ apt-get -s install w3c-sgml-lib
NOTE: This is only a simulation!
      apt-get needs root privileges for real execution.
      Keep also in mind that locking is deactivated,
      so don't depend on the relevance to the real current situation!
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

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: w3c-markup-validator (not installed)
ProcVersionSignature: Ubuntu 2.6.38-10.44-generic 2.6.38.7
Uname: Linux 2.6.38-10-generic x86_64
Architecture: amd64
Date: Tue Jun 28 15:18:17 2011
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Beta amd64 (20110413.1)
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: w3c-markup-validator
UpgradeStatus: No upgrade log present (probably fresh install)