docx4all is an open source cross platform WYSIWYG editor for docx files, written in Java Swing and JavaFX Script, released under the GPL. See SystemRequirements.

Docx is its native file format, so users won't have to worry about lossy round tripping when opening docx files.

The editor can run both as a desktop application, or from within a web browser.

See further http://www.docx4java.org/trac/docx4all and http://www.docx4java.org/forums/docx4all-f7/