Ok! It's for Google Chrome this issue. Safari, IE, FF, Opera work fine.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>J</title>
<script type="text/javascript">
function changeStyle()
{
var x=document.getElementById("myframe");
var y=(x.contentWindow || x.contentDocument);
if (y.document)
y=y.document;
y.body.style.backgroundColor="#0000ff";
}
</script>
</head>
<body onload="changeStyle()">
<iframe id="myframe" src="js1.htm">
<p>Your browser does not support iframes.</p>
</iframe>
<br /><br />
<input type="button" onclick="changeStyle()" value="Change background color" />
</body>
</html>