Or, to be more precise, Emacs/nxml-mode (editing an XML document and
validating it against a RELAX NG compact syntax grammar as you go); there is
also Emacs/psgml-mode for editing an XML document and validating it against a
DTD.

I think what has been posted here about Emacs is correct  it is in some ways
the best editor, for most anything. But it comes with a serious downside,
too: it is not like any other tool you use or will use, and thus it is very
difficult to learn to use well.

But MSMcQs point about macro language is a very good one, IMHO. (I may be
mildly biased having also grown up on XEDIT & Rexx : -)

All that said, I also use oXygen. It is not perfect and has a lot of features
I could not care less about, but I have never seen a commercial XML editor
with a price/feature ratio anywhere near as good as oXygens.

________________________________
Relax-ng mode.