<div id="infodiv" style="position:absolute;width:200px;height:150px;visibility:hidden;">Hallo</div>
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return (document.compatMode && document.compatMode!="BackCompat")? document.documentElement : document.body
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function showinfo(msg) {
document.getElementById("infodiv").innerHTML = "<h3>" + msg + "</h3>";
divWidth = document.getElementById('infodiv').style.width.replace("px" , "");
divHeight = document.getElementById('infodiv').style.height.replace("px" , "");
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document.getElementById("infodiv").style.top = iebody().scrollTop + iebody().clientHeight / 2 - divHeight / 2 + "px";
document.getElementById("infodiv").style.left = (iebody().clientWidth / 2) - (divWidth / 2) + "px";
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document.getElementById("infodiv").style.top = window.pageYOffset + window.innerHeight / 2 - divHeight / 2 + "px";
document.getElementById("infodiv").style.left = (window.innerWidth / 2) - (divWidth / 2) + "px";
}
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<a href="http://www.google.de" onmouseover="JavaScript:showinfo('Sie werden zu Google gelangen')" onmouseout="JavaScript:hideinfo()">Maus hier drüber</a>