You don't have to put anything in a head tag, you don't even need a head tag, but it's a good idea to have one and to at least add a charset meta tag. Everything is optional, though.
The entire point of it is for you to share metadata about your page with the browser.
My favorite weird thing that happens without certain <head> tags is the ten billion attempts to get a favicon. I don't know if browsers still try all those silly favicon requests if none is defined, but it was saddening to watch that grow to be wide spread previously.
Yes, they all still issue several favicon requests, so far as I'm aware. Having one, instead of returning a 404, can speed up the page load. Which makes 0 sense.
You don't have to put anything in a head tag, you don't even need a head tag, but it's a good idea to have one and to at least add a charset meta tag. Everything is optional, though.
The entire point of it is for you to share metadata about your page with the browser.