Serverside includes are a thing in Apache or nginx. If you don’t mind a little coding, it wouldn’t take long to build a tool that lets you store the bodies as

Serverside includes are a thing in Apache or nginx. If you don’t mind a little coding, it wouldn’t take long to build a tool that lets you store the bodies as template files, wraps them in the header/footer, and minifies them. Even less effort is pandoc’s standalone mode which lets you provide a wrapper template.