如何让按钮与div对齐

<style>
    .button {
        display: block;
        float: left;
    }
    .div {
        display: block;
        clear: both;
    }
</style>

<div class="button">
    <button>按钮</button>
</div>
<div class="div">
    <p>这是一个div。</p>
</div>

<style>
    .container {
        display: flex;
        align-items: flex-start;
    }
</style>

<div class="container">
    <button>按钮</button>
    <div>
        <p>这是一个div。</p>
    </div>
</div>