Xopus, the friendly XML editor, that's not as friendly as it used to be...

For a long time, I have wished for a solid, slick, browser-based xml editor and the only thing that seems to meet that definition is Xopus.
It appears that a few years back it was Open Source and moving towards cross-browser compatibility, but at some point went closed source IE-only (and a tad expensive). To quote a character from A Mighty Wind... "Wha' Happened?".