jEdit is one of the view free or open source tools that includes editing and validation tools for XML, XSLT and XQuery coding, and is sometimes proposed as an alternative to Oxygen when a completely Open pipline of tools is desirable. It does not have all of the functionality or support comparable to a paid tool, of course, and requirely slightly more setup and maintenance on the part of the user.
Taken from the project website (accessed 2019-08-23):
jEdit is a mature programmer's text editor with hundreds (counting the time developing plugins) of person-years of development behind it. To download, install, and set up jEdit as quickly and painlessly as possible, go to the Quick Start page. While jEdit beats many expensive development tools for features and ease of use, it is released as free software with full source code, provided under the terms of the GPL 2.0.
Some of jEdit's features include:
- Written in Java, so it runs on Mac OS X, OS/2, Unix, VMS and Windows.
- Built-in macro language; extensible plugin architecture. Hundreds of macros and plugins available.
- Plugins can be downloaded and installed from within jEdit using the "plugin manager" feature.
- Auto indent, and syntax highlighting for more than 200 languages.
- Supports a large number of character encodings including UTF8 and Unicode.
- Folding for selectively hiding regions of text.
- Word wrap.
- Highly configurable and customizable.
- Every other feature, both basic and advanced, you would expect to find in a text editor. See the Features page for a full list.