Extenting 4.2.x share your can extend the FTL templates by region or markup.
I have done this a couple of times, using regions and markup defined in files.
In a case i wanted to add a JS file to the loading of actions-common.get.html.ftl, but the file does not have a markup and is reused in many places, so not really mapped to just one region.
Therefore my question is, is there a default markup for every FTL file, like id="default", which you could hook into (or another approach)?