使用：focus选择器选择具有焦点的元素。您可以尝试运行以下代码来实现：focus选择器
<!DOCTYPE html>
<html>
<head>
<style>
input:focus {
background-color: green;
}
</style>
</head>
<body>
<form action = "">
Subject <input type = "text" name = "subject"><br>
Student: <input type = "text" name = "student"><br>
Age: <input type = "number" name = "age"><br>
<input type = "submit" value = "Submit">
</form>
</body>
</html>