DTD syntax highlighting for Notepad++ (http://notepad-plus-plus.org)
- Download the language file (Save Link As...) from https://github.com/nakohdo/NPP.DTD/raw/master/userDefineLang_DTD.xml
- In Notepad++, import the file via Language > Define your language... > Import....
For detailed information on UDL see User Defined Language Online Documenation.