A map paint style that highlights orphan nodes (those that have no tags and are not part of a way) and dims any other elements.
Adding the map paint style to JOSM:
First, make the "Map paint style" window visible:
Add a new map paint style by clicking on the marked button:
Then click on the marked button as seen below:
Add a name and a path to where you saved the "orphan_nodes.mapcss" file
The map paint style should then appear in the list. Check it and uncheck any other map paint style you can see there.
TaDAAAAM! You did it. Now be careful and think twice before deleting orphan nodes (more instructions here: http://blog.improve-osm.org/en/how-to-deal-with-orphan-nodes-in-osm/)