CSS3 opacity Property

opacity: number|initial|inherit;

div {
    opacity: 0.5;
}

function myFunction(x) {
// Return the text of the selected option
    var opacity = x.options[x.selectedIndex].text;
    var el = document.getElementById("p1");
    if (el.style.opacity !== undefined) {
        el.style.opacity = opacity;
    } else {
        alert("Your browser doesn't support this.");
    }
}