Question

I need to detect (in jquery) if a marker:

<div id="marker"></div>

has been reached/hit when scrolling down on a web page.

Answer 1

Basically you need this:

DEMO

function onScreen(){
  var markerPos = $('#marker').offset().top;
  var winScroll = $(document).scrollTop();
  var winH      = $(window).height();

  if( winScroll+winH >= markerPos){
    console.log('VISIBLE!');
  } 
}

$(window).on('scroll resize',function(){ 
    onScreen();  
});