Подключаемый JS
var opacity = 0;
var start = 0;
function openauthorize()
{
if(opacity>1) clearInterval(setIN);
opacity=opacity+0.1;
document.getElementById('df').innerHTML=opacity;
}
function closeauthorize()
{
opacity=opacity-0.1;
document.getElementById('df').innerHTML=opacity;
}
HTML страница
...
<div id='df'></div>
<span onClick='var setIN=setInterval(openauthorize,10)'>Плюс</span>
<span onClick='setInterval(closeauthorize,10)'>Минус</span>
...