kurganoffiv,
offsetLeft надо предварительно закешировать.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#testDIV{
position: relative;
}
</style>
<script>
document.addEventListener("DOMContentLoaded", function() {
var testDiv = document.querySelector("#testDIV");
var childTestDiv = testDiv.children;
[].map.call(childTestDiv, function(child) {
var leftPos = child.offsetLeft;
child.style.left = leftPos + "px";
return child
}).forEach(function(child) {
child.style.position = "absolute"
})
});
</script>
</head>
<body>
<div id="testDIV">
<span>SPAN 1</span>
<span>SPAN 2</span>
</div>
</body>
</html>