<html><body><h1>input placeholder 属性</h1><form action="/demo/action_page.php"> <label for="phone">请输入电话号码：</label><br><br> <input type="tel" id="phone" name="phone" placeholder="138-1234-5678" pattern="[0-9]{3}-[0-9]{4}-[0-9]{4}"><br><br> <small>格式：138-1234-5678</small><br><br> <input type="submit"></form></body></html>