The scrolling viewport is represented by
<html>) in standards mode or
<body> in quirks mode. (Quirks mode emulates the document rendering of Navigator 4 and Explorer 5.)
body.scrollTop to represent the viewport's scroll position in both modes, which is wrong. It sounds like they want to fix this so they're encouraging authors to script for the standard behavior.
I don't think you need to change your code. There's nothing wrong with using
body.scrollTop in standards mode so long as you understand it represents the scroll position of
body only (typically
0, unless you've given
body a scroll box).
You can see the warning by executing
document.body.scrollTop in the console:
body.scrollTopis deprecated in strict mode. Please use
documentElement.scrollTopif in strict mode and
body.scrollToponly if in quirks mode.