Monobook.css contains the following definition for the body tag, in the main
<body class="ns-0 ltr">
In other namespaces the 0 is replaced by the relevant number, allowing
namespace-specific CSS rules to be applied to page elements.
The Classic, Nostalgia and Cologne Blue skins (and possibly others) do not
include this definition, so namespace customisation is effectively disabled for
This is particularly important for mediawiki.org, where we use
namespace-specific CSS to differentiate between content released under GFDL and
our public domain content.