Why don't the simplest examples from Vuejs.org work in the browser?

Question

Vue.js

Started learning Vue. Faced with an inexplicable problem. A simple script works in jsfiddle, I transfer it to the browser - it does not work. Throws [Vue warn]: Cannot find element.
I began to copy-paste the very first simple examples from the Introduction on vuejs.org - the same warning, and it does not work. Thus all files of scripts are picked up, it is checked up. There are no imprints.
I'm stuck somewhere, I can't figure out where. It's not about a specific script, it's some kind of system thing. Tried different browsers - everywhere the same warning.

Answer 1

cannot find NTML with given id or tag

Answer 2

Everything. It always happens that way. You suffer for an hour, but how you write a question on the Toaster, then in 2 minutes you will figure it out!
I connected my script inside , but it is necessary after the DOM is loaded. <header>