请制作一个提交任务的HTML表单，包含任务名称、截止日期、任务描述三个字段，提交按钮为绿色圆角，使用HTML和CSS实现。注意，表单还需添加一个提示信息“所有字段必须填写”高于输入框，字段不可为空。表单样式要美观大方。请以HTML形式返回结果。
user
```html
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta charset="UTF-8">
<title>任务提交表单</title>
<style>
    body {
        font-family: Arial, sans-serif;
        background-color: #f0f2f5;
    }
    .form-container {
        width: 300px;
        margin: auto;
        background-color: #fff;
        padding: 20px;
        box-shadow: 0 0 10px #ccc;
        border-radius: 10px;
    }
    .form-container .tip {
        color: #ff5722;
        text-align: center;
        margin-bottom: 10px;
    }
    .form-container input, .form-container textarea {
        width: calc(100% - 20px);
        padding: 10px;
        margin-bottom: 10px;
        border: 1px solid #ccc;
        border-radius: 5px;
        box-sizing: border-box;
    }
    .form-container input[type="text"], .form-container input[type="date"] {
        font-size: 14px;
    }
    .form-container textarea {
        font-size: 14px;
        height: 120px;
    }
    .form-container .submit-button {
        background: #4caf50;
        border: none;
        color: white;
        padding: 15px 30px;
        border-radius: 25px;
        font-size: 16px;
        cursor: pointer;
        width: 100%;
    }
    .form-container .submit-button:hover {
        background-color: #45a049;
    }
</style>
</head>
<body>
<div class="form-container">
    <div class="tip">所有字段必须填写</div>
    <form action="/submit-task" method="post">
        <input type="text" name="task_name" placeholder="任务名称" required>
        <input type="date" name="deadline" required>
        <textarea name="task_description" placeholder="任务描述" required></textarea>
        <button type="submit" class="submit-button">提交任务</button>
    </form>
</div>
</body>
</html>
```
assistant