A simple bookmarklet to hide your YouTube controls. Click on it to hide the controls, click on it again to show the controls again.
javascript:var s=document.getElementById("stylehidecontrols");if(s){s.remove();}else{s=document.createElement("style");s.id="stylehidecontrols";var r="#movie_player .ytp-gradient-top, #movie_player .ytp-gradient-bottom, #movie_player .ytp-chrome-top, #movie_player .ytp-chrome-bottom{display:none !important}";s.appendChild(document.createTextNode(r));document.body.appendChild(s);}void 0;
(The code is an adapted version of jscher2000's code on the Mozilla support site)