As an alternative to the closed-source XML Editor we have been using
earlier, there is now a tool named [VEX|http://www.eclipse.org/vex/] based on
[Eclipse|http://www.eclipse.org].

Using Vex, one can edit the xml tree in a visually clean way, using CSS to
style each element as wanted.

Documentation can be found [here|http://wiki.eclipse.org/Vex/UserGuide].

The Vex plugin for Eclipse seems to still be under slow development (last commit
was in 2016), but there are bugs, and so far the user experience has been less
than great. It could be worth investigating - it is the only open-source,
CSS-styled Xml editor out there, and it is also cross-platform. If desirable, we
could add some man power to the project, to get it up to speed and to fit our
needs.