@namespace namespace_name url
namespace_name|selector {}

Defines a namespace foo within CSS:

@namespace foo "http://www.example.com" 

selections based on presence (or not) of namespace foo

foo|h1  {color:blue}   h1 in namespace foo
*|h1    {color:yellow} h1 in any namespace
|h1     {color:red}    h1 not declared in any namespace
h1 [*|attr]   matches attribute attr in any namespace
h1 [foo|attr]    matches attribute attr declared in  namespace foo
h1 [attr]     matches attribute attr not declared in any namespace