Hide/Toggle ALL YouTube Player Controls & UI

toggle-youtube-controls.js

// ==UserScript==
// @name         Hide/Toggle YouTube Player Controls/UI
// @description  Hide/Toggle YT Player Controls & UI with the "H" key
// @version      0.1
// @match        https://www.youtube.com/watch?*
// ==/UserScript==

var css = "#movie_player > :not(.html5-video-container) { display: none !important }"

var style = document.createElement('style')

style.textContent = css
style.id = 'hide_controls'
document.head.appendChild(style)
// style must be disabled AFTER document insertion or else it will not be disabled
style.disabled = true // disabled by default?

window.addEventListener('keydown', function(e) {
  if (e.key.toLowerCase() === 'h')
    style.disabled = (! style.disabled)
})

works like a charm, thanks!