I am going to add similar support for attribute names and possibly values though this will depend on NetBeans bug #198223. You can vote for it and hopefully it will be integrated into next NetBeans version.
On a similar subject: XML files describing layouts can get some coding support too. First changes are already there. A subset of available attribute names is shown in code completion window. Full set of attributes, tag names and other items should follow.
As usually everything is available from our continuous build first.