​x
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<style type="text/css">
img[alt]
{
border: 5px solid red;
}
</style>
</head>
​
<body>
<h1>可以应用样式：</h1>
<img src="/i/w3school_logo_black.gif" alt="W3School Logo" />
​
<hr />
​
<h1>无法应用样式：</h1>
<img src="/i/w3school_logo_black.gif" />
</body>
</html>
​