Excel makes proper CSVs insofar as when you say that you allow people to import CSV files into your application, the vast vast majority will be using Excel to c

Excel makes proper CSVs insofar as when you say that you allow people to import CSV files into your application, the vast vast majority will be using Excel to create / manipulate those CSV files (and if they don't, whatever tool they DO use will have made efforts to work consistently with Excel). What any other spec actually says is irrelevant. For most non-technical users, anything Excel outputs as a CSV is a CSV file (and anything you output that doesn't work with Excel is not)