HTML5 page visibility API example http://ssbits.wordpress.com/category/html5/

gistfile1.txt

//Call this function which will do the work of rendering if needed
visChange();

//check if the div_blogroll is visible on the page, if so, render it
function visChange() {
    var divBlog = document.getElementById('div_blogroll');
    if (divBlog) {
        if (!isHidden())
            doLoadBlogRoll();
    }
}

//pagevisibility API
function getHiddenProp() {
    var prefixes = ['webkit', 'moz', 'ms', 'o'];
    // if 'hidden' is natively supported just return it
    if ('hidden' in document) return 'hidden';
    // otherwise loop over all the known prefixes until we find one
    for (var i = 0; i < prefixes.length; i++) {
        if ((prefixes[i] + 'Hidden') in document)
            return prefixes[i] + 'Hidden';
    }
    // otherwise it's not supported
    return null;
}
 
//helper function
function isHidden() {
    var prop = getHiddenProp();
    if (!prop) return false;
    return document[prop];
}
 
//use the property name to generate the prefixed event name
var visProp = getHiddenProp();
if (visProp) {
    var evtname = visProp.replace(/[H|h]idden/, '') + 'visibilitychange';
    document.addEventListener(evtname, visChange);
}