What do you use to get element’s offset? Looping through its parents and summing offsetLeft/offsetTop numbers? I bet you had problems with it. Well, I did, especially when additional positioning context was generated by an element with position!=static.
10 years ago in IE5.0 Microsoft invented a better way - getBoundingClientRect() method.
It’s been popularised by PPK, specified in cssom-view spec, copied by Firefox 3, Opera 9, Chrome 2, Safari; used in jQuery.
element.getBoundingClientRect() basically returns the object containing this element’s coordinates according to the window element. So you don’t have to do a loop, just call this method once and get the results. The only thing it doesn’t count for is scrolling offset, which can be calculated easily.
Extremely useful, thanks Microsoft IETeam!