I built the following YAML, and it appears to work really well.
https://github.com/sjackson0109/ZabbixTemplates/blob/main/scripts/validate_zabbix_template.py

Could someone support me in 'pointing out the code in the zabbix frontend' ... that actually performs the YAML validation in the GUI? I would like to make this offline variant as accurate as possible; and maybe improve on it...

Please be advised that this section of the tracker is for feature requests only. The case you have submitted can not be qualified as one, so please reach out to [email protected] for commercial support (https://zabbix.com/support) or consultancy services. Alternatively, you can also use our IRC channel or community forum (https://www.zabbix.com/forum) for assistance.

With that said, we are closing this ticket. Thank you for understanding.

vjaceslavs  As a minimum, before closing ANY ticket, you should explain in the comments, your reasoning for updating the ticket.  Marking a ticket as "won't do" is just, frankly, obnoxious. 
I'm trying to offer some level of community offerings here, and going via the formal channels to do that - and i get an immediate rejection with no explaination - this is embarrasing for you, and will only deter people like me from contributing!

sjackson0109, my previous comment had an invalid security level set, making it invisible (sorry, my bad).

I hope it is now visible and clarifies why this ticket was closed.