- support for DocBook
- preferably to work on any operating system but a must is Mac OS X
XPontus XML Editor is a simple XML Editor oriented towards text editing. It can perform validation(DTD, XML Schema, Relax NG, Batch XML validation), XSL transformations(HTML, XML, PDF, SVG), schema/DTD generation, XML/DTD/HTML/XSL code completion, code formatting and much more. The brand new version is now plugin based to solve some limitation issues.
XPontus aims to become the free alternative to commercial XML IDES such as XMLSPy or Oxygen XML Editor, or Exchanger XML Editor without the Entreprise features for now. The application tries to provide you most of the features you would expect to see in a fully bloated XML IDE, in a familiar and conviviable manner.
You can use the XMLmind XML Editor. Here is how it compares to your requirements:
free: yes, as in "gratis"; it is free for personal use and to work on open source projects (see), but it is not open source software itself
cross-platform: yes – MacOS, Windows, Linux and anything running OpenJDK 14
Docbook support: yes – try the example Docbook file from the "recently opened" section of the "File" menu, right after installation
WYSIWYG: yes – choose any stylesheet-enabled view from the "View" menu