> Does it really make sense to make this a customizable option?  How
> would an average user choose a value for this option?
>
> I think EWW should decide by itself what header to send.

I agree, it is not something most people will want to change. If they
really want to change it though, they can just do (setq
eww-accept-content-types ...). I've changed the patch to use defvar
instead of defcustom.