<!DOCTYPE html>
<html>
  <head>
    <title>background-position-x: initial on top of 4-value background-position</title>
    <style>
body { margin: 0 }
.ref {
   height: 100px;
   width: 100px;
   top: 100px;
   left: 0;
   background-color: green;
   position: absolute;
}
    </style>
  </head>
  <body>
    <div class="ref"></div>
  </body>
</html>