XML Notepad is not available for Mac but there are plenty of alternatives that runs on macOS with similar functionality. The best Mac alternative is Vim, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to XML Notepad and many of them are available for Mac so hopefully you can find a suitable replacement. Other interesting Mac alternatives to XML Notepad are Geany (Free, Open Source), GNU Emacs (Free, Open Source), CudaText (Free, Open Source) and UltraEdit (Paid).
CudaText is a cross-platform text editor, written in Lazarus. It starts quite fast (0.5 sec with ~30 plugins on CPU Intel Core i3 3Hz). It is extensible by Python add-ons (plugins, linters, code tree parsers, external tools).
ReText is a simple text editor for Markdown and reStructuredText documents with syntax highlighting and optional live preview. It is written in Python using Qt libraries, able to run on any platforms (Linux and BSD are officially supported).
Eddie is a programmers editor for Mac OSX and Gnome Linux. Inspired by the Macintosh Programmer's Workshop, Eddie brings some of it's powerful features into a modern lightweight and capable programmer's editor.