A simple little function to get the position of an element with respect to the upper top left of the viewport. Uses getBoundingClientRect() for today's browsers and falls back to an older method for older browsers.

getPosition.js

// Get position of element from top of viewport.

function getPosition(element) {
	// Modern browsers.
	if(element.getBoundingClientRect) {
		var clientRect = element.getBoundingClientRect();

		return {
			x: Math.floor(clientRect.left),
			y: Math.floor(clientRect.top)
		};
	}

	// Old browsers.
  	var xPosition = 0;
  	var yPosition = 0;

	while(element) {
		xPosition += (element.offsetLeft - element.scrollLeft + element.clientLeft);
		yPosition += (element.offsetTop - element.scrollTop + element.clientTop);
		element = element.offsetParent;
	}

	return { x: xPosition, y: yPosition };
}