BackCompat = Back Compatible = 後方互換モードだとdocument.bodyをみないと値が取れないためrootを設定。

JS: Detect size and scroll

var root = (document.compatMode === "BackCompat") ? document.body : document.documentElement;
console.log([
  root.clientHeight,//height of the visible area
  root.clientWidth,//width of the visible area
  root.scrollHeight,//document height including hidden(scroll) area
  root.scrollWidth,//document width including hidden(scroll) area
  window.pageYOffset || root.scrollTop,
  window.pageXOffset || root.scrollLeft,
]);