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前言：要想改變表單元素樣式，一般都是先在CSS檔案中定義想讓表單元素變成的樣式，然後在需要的地方給該元素的className屬性添加上樣式名稱。注意，兩個樣式名稱間要有空格。樣本如下：
=>CSS檔案
body {
color:#000;
// 黑色;
background-color:#FFF; // 白色;
}
input.invalid {
background-color:#FF9; // 黃色;
border: 2px red inset;
}
label.invalid {
color: #F00; // 紅色;
font-weight: bold;
}
=>HTML檔案：
<!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" />
<link type="text/css" rel="stylesheet" href="css.css" />
<title>HTML與CSS</title>
<script language="javascript">
function btnEvent(btn) {
var name = document.getElementById("userName");
name.focus();
var oldclass = name.className;
console.log(oldclass);// reqd;
oldclass ="invalid " + oldclass;
console.log(oldclass);// invalid reqd;
name.className = oldclass;
}
</script>
</head>
<body>
<form action="#">
<table>
<tr><td>名字：</td><td><input type="text" size="30" id="userName" class="reqd" /></td></tr>
<tr><td>密碼：</td><td><input type="text" size="30" id="passwd1" class="reqd" /></td></tr>
<tr><td>密碼確認：</td><td><input type="text" size="30" id="passwd2" class="reqd passwd1" /></td></tr>
<tr><td></td><td><input type="button" value="改變樣式" onclick="btnEvent(this)"</td></tr>
</table>
</form>
</body>
</html>
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