Edit in JSFiddle

var current = null;
document.body.addEventListener('click', function(ev) {
    if (ev.target.tagName === 'BUTTON') {
        if (current) {
            current.classList.remove('focus');
        }
        current = ev.target;
        current.classList.add('focus');
    }
}, false);

button {
    display: block;
}
.focus {
    background: red;
}

<button>Click me</button>
<button>No, me</button>
<button>Seriously, I click much better</button>
<button>Don't listen to that guy</button>
<button>I clicked before it was cool</button>
<button>Don't click me</button>