<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Counter</title>
<script type="text/javascript" language="javascript">
var secCount = 0;
function Timer()
{
secCount+=5;
document.getElementById("counter").innerHTML = secCount;
window.setTimeout("Timer()", 5000);
}
</script>
</head>
<body onload="window.setTimeout('Timer()', 5000);">
<h3 style="text-align: center;">Прошло <span id="counter" style="color: red; font-size: 150%;">0</span> секунд.</h3>
</body>
</html>