使用CSS：有效选择器为所有<input>元素设置有效值的样式。
您可以尝试运行以下代码来实现：valid选择器
<!DOCTYPE html>
<html>
<head>
<style>
input:valid {
background: red;
color: white;
}
</style>
</head>
<body>
<h2>Heading</h2>
<input type = "email" value = "amit@example.com">
<p>The style (red color background) appears if you type a relevant/ valid email address.</p>
</body>
</html>