var element = document.elementFromPoint(x, y);
在给定的坐标点处的顶端 Element（译者注：如果元素层叠的话，返回最上层的元素）。
If the element at the specified point belongs to another document (for example, an iframe’s subdocument), the subdocument’s parent element is returned (the iframe itself). If the element at the given point is anonymous or XBL generated content, such as a textbox’s scroll bars, then the first non-anonymous ancestor element (for example, the textbox) is returned.
