Check if element is visible on viewport

isOnViewPort

        var isOnViewPort = (function() {
            var compatMode = document.compatMode;
            function posY(elm) {
                var test = elm, top = 0;

                while(!!test && test.tagName.toLowerCase() !== "body") {
                    top += test.offsetTop;
                    test = test.offsetParent;
                }

                return top;
            }

            function viewPortHeight() {
                var de = document.documentElement;

                if(!!window.innerWidth){
                    return window.innerHeight;
                }else if(compatMode == 'CSS1Compat'){ 
                    return de.clientHeight;
                }else if(compatMode == 'BackCompat'){
                    return document.body.clientHeight;
                }
            }

            function scrollY() {
                if( window.pageYOffset ) { 
                    return window.pageYOffset;
                }else if(compatMode == 'CSS1Compat'){
                    return document.documentElement.scrollTop;
                }else if(compatMode == 'BackCompat'){
                    return document.body.scrollTop;
                }
            }

            function _isOnViewPort( elm ) {
                var vpH = viewPortHeight(), // Viewport Height
                    st = scrollY(), // Scroll Top
                    y = posY(elm);

                return st < y && y - st < vpH;
            }
            return _isOnViewPort;
        })();