The default value is visible, which allows overflowing content to be rendered outside the containing block.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<style type="text/css">
* .ex1 {
width: 100px;
height: 30px;
background: yellow;
}
</style>
</head>
<body>
<div class="ex1"> this is a test. this is a test. this is a test. this is a test.
this is a test. this is a test. this is a test. this is a test.
</div>
</body>
</html>