<html><head><style>input:focus { background-color: yellow;}</style></head><body><h1>:focus 实例</h1><p>点击（或通过 Tab 键聚焦到）输入框可查看黄色背景：</p><form><label for="name">姓名：</label><br><input id="name" name="name" type="text" /><br><label for="email">邮箱：</label><br><input id="email" name="email" type="email" /><br><label for="country">国家：</label><br><input id="country" name="country" type="text" /></form></body></html>