I understand Mailman 3 doesn't want to support HTML Emails, This is perfectly legitimate, sadly however, This is a requirement of my client, Which is in this case the Ministry of Education of Israel.
This is not the first time a client of mine requires this, This is also was required and implemented in Mailman 2 couple of years ago by some other client (big college).
So starting to implement it, I found this article: https://blog.hubspot.com/marketing/plain-text-vs-html-emails-data
which also claims that HTML Emails is prefered by many users (although I understand the problematic nature of HTML Emails)
Could You give me some directions on how to go about implementing this ?