After looking at https://cyh.herokuapp.com/cyh and https://www.owasp.org/index.php/List_of_useful_HTTP_headers and checking what we currently use as/in HTTP header fields in themes and other places I think it's time to update the header.
The idea is to create a new header FTL template in framework/image and to include it "where needed" and possibly refactor what currently exist.
As there are many ways to define "where needed". I decided to keep things simple. There are (case insensitive) 324 occurences of "<head>", 112 occurences of "<html" but only 16 occurences of "<html lang=" and only 12 in FTL templates (others are in jQuery files)
Most of the 112 occurences of "<html" belong to email templates. So I will focus on the not email templates cases among the 12 in FTL templates.