What return height size by IE, backCompat IE?

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var body = document.body,

html = document.documentElement;

//document.compatMode === backCompat
body.clientHeight //=> viewport size.
body.scrollHeight //=> contents size.
body.offsetHeight //=> viewport size.

html.clientHeight //=> 0;
html.scrollHeight //=> viewport size.
html.offsetHeight //=> viewport size.

//document.compatMode === CSS1compat
body.clientHeight //=> contents size.
body.scrollHeight //=> contents size.
body.offsetHeight //=> contents size.

html.clientHeight //=> viewport size.
html.scrollHeight //=> contents size.(larger)
html.offsetHeight //=> viewport size.