Obtener las coordenadas de un elemento html

// ===================================================================
// Author: Matt Kruse <matt@mattkruse.com>
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/* 
AnchorPosition.js
Author: Matt Kruse
Last modified: 10/11/02

DESCRIPTION: These functions find the position of an <A> tag in a document,
so other elements can be positioned relative to it.

COMPATABILITY: Netscape 4.x,6.x,Mozilla, IE 5.x,6.x on Windows. Some small
positioning errors - usually with Window positioning - occur on the 
Macintosh platform.

FUNCTIONS:
getAnchorPosition(anchorname)
  Returns an Object() having .x and .y properties of the pixel coordinates
  of the upper-left corner of the anchor. Position is relative to the PAGE.

getAnchorWindowPosition(anchorname)
  Returns an Object() having .x and .y properties of the pixel coordinates
  of the upper-left corner of the anchor, relative to the WHOLE SCREEN.

NOTES:

1) For popping up separate browser windows, use getAnchorWindowPosition. 
   Otherwise, use getAnchorPosition

2) Your anchor tag MUST contain both NAME and ID attributes which are the 
   same. For example:
   <A NAME="test" ID="test"> </A>

3) There must be at least a space between <A> </A> for IE5.5 to see the 
   anchor tag correctly. Do not do <A></A> with no space.
*/ 

// getAnchorPosition(anchorname)
//   This function returns an object having .x and .y properties which are the coordinates
//   of the named anchor, relative to the page.
function getAnchorPosition(anchorname) {
 // This function will return an Object with x and y properties
 var useWindow=false;
 var coordinates=new Object();
 var x=0,y=0;
 // Browser capability sniffing
 var use_gebi=false, use_css=false, use_layers=false;
 if (document.getElementById) { use_gebi=true; }
 else if (document.all) { use_css=true; }
 else if (document.layers) { use_layers=true; }
 // Logic to find position
  if (use_gebi && document.all) {
  x=AnchorPosition_getPageOffsetLeft(document.all[anchorname]);
  y=AnchorPosition_getPageOffsetTop(document.all[anchorname]);
  }
 else if (use_gebi) {
  var o=document.getElementById(anchorname);
  x=AnchorPosition_getPageOffsetLeft(o);
  y=AnchorPosition_getPageOffsetTop(o);
  }
  else if (use_css) {
  x=AnchorPosition_getPageOffsetLeft(document.all[anchorname]);
  y=AnchorPosition_getPageOffsetTop(document.all[anchorname]);
  }
 else if (use_layers) {
  var found=0;
  for (var i=0; i<document.anchors.length; i++) {
   if (document.anchors[i].name==anchorname) { found=1; break; }
   }
  if (found==0) {
   coordinates.x=0; coordinates.y=0; return coordinates;
   }
  x=document.anchors[i].x;
  y=document.anchors[i].y;
  }
 else {
  coordinates.x=0; coordinates.y=0; return coordinates;
  }
 coordinates.x=x;
 coordinates.y=y;
 return coordinates;
 }

// getAnchorWindowPosition(anchorname)
//   This function returns an object having .x and .y properties which are the coordinates
//   of the named anchor, relative to the window
function getAnchorWindowPosition(anchorname) {
 var coordinates=getAnchorPosition(anchorname);
 var x=0;
 var y=0;
 if (document.getElementById) {
  if (isNaN(window.screenX)) {
   x=coordinates.x-document.body.scrollLeft+window.screenLeft;
   y=coordinates.y-document.body.scrollTop+window.screenTop;
   }
  else {
   x=coordinates.x+window.screenX+(window.outerWidth-window.innerWidth)-window.pageXOffset;
   y=coordinates.y+window.screenY+(window.outerHeight-24-window.innerHeight)-window.pageYOffset;
   }
  }
 else if (document.all) {
  x=coordinates.x-document.body.scrollLeft+window.screenLeft;
  y=coordinates.y-document.body.scrollTop+window.screenTop;
  }
 else if (document.layers) {
  x=coordinates.x+window.screenX+(window.outerWidth-window.innerWidth)-window.pageXOffset;
  y=coordinates.y+window.screenY+(window.outerHeight-24-window.innerHeight)-window.pageYOffset;
  }
 coordinates.x=x;
 coordinates.y=y;
 return coordinates;
 }

// Functions for IE to get position of an object
function AnchorPosition_getPageOffsetLeft (el) {
 var ol=el.offsetLeft;
 while ((el=el.offsetParent) != null) { ol += el.offsetLeft; }
 return ol;
 }
function AnchorPosition_getWindowOffsetLeft (el) {
 return AnchorPosition_getPageOffsetLeft(el)-document.body.scrollLeft;
 } 
function AnchorPosition_getPageOffsetTop (el) {
 var ot=el.offsetTop;
 while((el=el.offsetParent) != null) { ot += el.offsetTop; }
 return ot;
 }
function AnchorPosition_getWindowOffsetTop (el) {
 return AnchorPosition_getPageOffsetTop(el)-document.body.scrollTop;
 }

Fuente: http://blog.crowe.co.nz/attachments/AnchorPosition.js