Set highest possible quaility and let space control pause and play.

youtube space handler

// ==UserScript==
// @name         Youtube space handler
// @version      1.1
// @description  Also sets highest possible quality for the video
// @author       Jesper Henrichsen
// @match        https://www.youtube.com/*
// ==/UserScript==
window.onkeydown = function(event){
    var bodyIsFocus = document.activeElement == document.getElementsByTagName('body')[0] || document.getElementsByTagName('video')[0];
    if (event.keyCode === 32 && bodyIsFocus) {
        if(event.preventDefault) {
           event.preventDefault();
            if( document.getElementsByClassName('ytp-button-play').length ) {
                document.getElementsByClassName('ytp-button-play')[0].click();
            } else if ( document.getElementsByClassName('ytp-button-pause').length ) {
               document.getElementsByClassName('ytp-button-pause')[0].click();
            }
        }
    } else if (event.keyCode == 70 && bodyIsFocus) {
      // pressed 'F' so going into fullscreen:
      document.getElementsByClassName('ytp-button-fullscreen-enter')[0].click();
    }
};


document.onreadystatechange = function() { 
    if (document.readyState =="complete") {
        var t = document.getElementsByClassName('ytp-drop-down'),
            q;
        for (var i = 0; i < t.length; i++) {
            if (t.item(i).getAttribute('aria-label') == 'Quality') {
                q = t.item(i);
                break;
            }
        };
        // the first is always the highest quality possible:
        q.getElementsByClassName('ytp-button')[0].click();
    }
};