[mootothemax]

very often without JS you pretty much get an empty HTML

The annoying thing is: if there were a header the browser could send to the server - "JS is turned off" - it'd be trivial to serve a pre-rendered full HTML response.

It's what plenty of e.g. Angular sites do to support the various search engines. Adding one more item to the list of "serve a prenderefndered view if X is true" checks wouldn't do any harm.