Thank you all for your suggestions.
Indeed, both codemirror ( http://codemirror.net/mode/xml/ ) and ace ( https://ace.c9.io/#nav=about ) look like excellent candidates.
Mark V. Sullivan
Sobek Digital Hosting and Consulting, LLC
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From: Sebastian Karcher <[log in to unmask]>
Sent: Thursday, September 29, 2016 11:11 PM
To: Code for Libraries; Mark V. Sullivan
Subject: Re: Online XML editing library (JQuery?)
we're also using codemirror, mentioned by Vincent, for the XML editor built into the Citation Style Language visual editor: http://editor.citationstyles.org/codeEditor/
I didn't build this, so can't speak to the difficulty of implementation, but it's certainly been stable and easy to maintain.