There is no mention of the XML schemas in the header.

[keltia]

It would be better to generate the full KML file with the schemas like in:

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:atom="http://www.w3.org/2005/Atom">

I note there is no support for the Google extensions (xmlns:gx="http://www.google.com/kml/ext/2.2"), would you be interested in these?

[pjsier]

Thanks for flagging this! I haven't looked deeply into the quick-xml library we're using here in a bit, but there's a KmlVersion enum that was set up to handle this in the future. Definitely open to thoughts or PRs here!