In HTTP/1.1 , when not specified , charset detault to ISO-8859-1

https://tools.ietf.org/html/rfc2616#section-3.7.1

> media subtypes of the "text"  type are defined to have a default charset value of "ISO-8859-1" when   received via HTTP

Zope adds a charset with default encoding (
https://github.com/zopefoundation/Zope/blob/2.13/src/ZPublisher/HTTPResponse.py#L512-L515 ) but does this only for text/ type, but we serve our javascript as application/javascript