Third party fitness tracking applications

hardcode the .CSV headers

I think it's definitely an option especially if that meant the file could be loaded by multiple different programs (we could have a drop-down for the CSV export format) - but when doing this initially it felt like CSV wasn't really a 'standard' in the way that GPX/KML was.

I guess it would be more feasible to feed (0,0) coordinates to the GPX file

Ahh, that's a good point about missing GPS data. It looks like the interface file currently just strips out data points where there is no GPS data: https://github.com/espruino/BangleApps/blob/master/apps/recorder/interface.html#L17

It'd be pretty easy to add an option that was something like 'use these coordinates when GPS unavailable', which might (hopefully) solve most issues for you.

TrainingPeaks doesn't support Bangle.JS's GPX-output, giving feedback that it's not formatted as expected by the standard.

Does it give any explanation of what's wrong? Do you have any example files that do work? Because I'm pretty sure the data was imported by Strava just fine (and I think I took the data format from an example Strava file).