A javascript snippet to create a square at the given coordinates.

squareatcoord.js

        javascript:(function (id,L,T,W,H,bgColor) {
	 if (document.getElementById) {
	  if (document.getElementById(id)) { document.body.removeChild(document.getElementById(id)); }

	  var highest_index = 0;
	  var elements = document.getElementsByTagName("*");
	  for (var i = 0; i < elements.length - 1; i++) {
	    //console.log(elements[i].style.zIndex);
	    if (parseInt(elements[i].style.zIndex) > highest_index) {
	        highest_index = parseInt(elements[i].style.zIndex);
	    }
	  }
	  highest_index = highest_index+1

	  var ST = 'position:absolute'
	  +'; left:'+L+'px'
	  +'; top:'+T+'px'
	  +'; width:'+W+'px'
	  +'; height:'+H+'px'
	  +'; clip:rect(0px,'+W+'px,'+H+'px,0px)'
	  +'; visibility:visible'
	  +'; z-index:'+highest_index
	  +(null==bgColor ? '' : '; background-color:'+bgColor);

	  var LR = '<DIV id='+id+' style="'+ST+'"></DIV>';

	  if (document.body) {
	   if (document.body.insertAdjacentHTML)
	       document.body.insertAdjacentHTML("BeforeEnd",LR);
	   else if (document.createElement
	        &&  document.body.appendChild) {
	    var newNode = document.createElement('div');
	    newNode.setAttribute('id',id);
	    newNode.setAttribute('style',ST);
	    document.body.appendChild(newNode);
	   }
	  }
	 }
	})('lyr_coord',400,150,10,10,'red');