Another bookmarklet that does the same thing, but a bit more polished:

    javascript:(function(){var e=document.body.style.cursor;document.body.style.cursor="crosshair";var t=document.createElement("div");var n="border:1px solid #3280FF;background-color:rgba(50,128,255,0.5);position:absolute;z-index:2147483647;display:none;";var r="pointer-events:none;";var i="transition:width 60ms,height 60ms,left 60ms,top 60ms;";n+=r+"-webkit-"+r+"-moz-"+r;n+=i+"-webkit-"+i+"-moz-"+i;t.setAttribute("style",n);document.body.appendChild(t);var s=null;var o=function(e){var n=e.target;if(n!==s&&n.parentNode){var r=n.getBoundingClientRect();var i=document.documentElement;var o=document.body;var u=i.clientTop||o.clientTop||0;var a=i.clientLeft||o.clientLeft||0;var f=window.pageYOffset||i.scrollTop||o.scrollTop;var l=window.pageXOffset||i.scrollLeft||o.scrollLeft;var c=l-a+r.left-1;var h=f-u+r.top-1;t.style.display="block";t.style.left=c+"px";t.style.top=h+"px";t.style.width=r.width+"px";t.style.height=r.height+"px";s=n}};var u=function(n){document.body.style.cursor=e;if(n.target.parentNode)n.target.parentNode.removeChild(n.target);if(t.parentNode)t.parentNode.removeChild(t);window.removeEventListener("click",u,false);window.removeEventListener("mouseover",o,false);n.stopPropagation();n.preventDefault()};window.addEventListener("mouseover",o,false);window.addEventListener("click",u,false)})();void(0)