lib-cov/jscoverage.js
function jscoverage_getViewportHeight() {
//#JSCOVERAGE_IF /MSIE/.test(navigator.userAgent)
  if (self.innerHeight) {
    // all except Explorer
    return self.innerHeight;
  }
  else if (document.documentElement && document.documentElement.clientHeight) {
    // Explorer 6 Strict Mode
    return document.documentElement.clientHeight;
  }
  else if (document.body) {
    // other Explorers
    return document.body.clientHeight;
  }
  else {
    throw "Couldn't calculate viewport height";
  }
//#JSCOVERAGE_ENDIF
}
function jscoverage_getViewportHeight() {
//#JSCOVERAGE_IF /MSIE/.test(navigator.userAgent)
  if (self.innerHeight) {
    // all except Explorer
    return self.innerHeight;
  }
  else if (document.documentElement && document.documentElement.clientHeight) {
    // Explorer 6 Strict Mode
    return document.documentElement.clientHeight;
  }
  else if (document.body) {
    // other Explorers
    return document.body.clientHeight;
  }
  else {
    throw "Couldn't calculate viewport height";
  }
//#JSCOVERAGE_ENDIF
}
http://www.quirksmode.org/viewport/compatibility.html