为什么input标签选择器不能居中？
<!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>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
#div1{width:300px;height:300px;background:red;margin:auto}
input {
margin:auto;
}
</style>
</head>
<body>
<input type="button" value="换色" />
<div id="div1">
</div>
</body>
</html>
