使用CSS设置placeholder的样式
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
<style>
#username::placeholder {
color: red;
}
</style>
</head>
<body>
用户名：<input id="username" type="text" placeholder="请输入用户名" />
</body>
</html>