JavaScript full screen relative to browser window, not monitor

I have made following function in JS:

function fullScreen(element) {
    if (element.requestFullScreen) {
        element.requestFullScreen();
    } else if (element.webkitRequestFullScreen) {
        element.webkitRequestFullScreen();
    } else if (element.mozRequestFullScreen) {
        element.mozRequestFullScreen();
    }

    link = document.getElementById("slideshow");
    link.removeAttribute("onclick");
    link.setAttribute("onclick", "cancelFullScreen()");
    }

This function lets me view the content full screen relative to the monitor. I can not find any information on how to make a full-screen relative to the browser window.

To be more specific, what I want concrete is to have the rest of the web site "blacked out" (like with opacity) while the picture is full screen inside of the browser (with or preferably without UI)