Formatting code in Notepad++

there is such a plugin as UniversalIndentGUI, it can be installed right from the plugin manager and has possibilities to reindent the most used programming languages.

ANSWER AS OF June 2019

Install the XML Tools plugin from the Plugin Admin (in Notepad++ 7.7 at least)

Then click Plugins -> XML Tools -> Pretty Print (XML Only with Line breaks)

That did it for me.

TextFX -> HTML Tidy -> Tidy: Reindent XML

Remember to have the HTML code selected before you do this.

If you go to TextFX menu and go to TextFX Edit, you will see a menu item Reindent C++ Code.

That will also format C# code.