<style>
    html, body { height: 100%; padding: 0; margin: 0;}
    /* flex 和 flex-grow  */
    .container {
        height: 100%;
        display: flex;
        flex-direction: column;
    }

    .box-A {
        height: 100px;
        background: #c0c0c0;
    }

    .box-B {
        flex-grow: 1;
        background: #ff6b81;
    }
</style>

<div class="container">
    <div class="box-A">xx</div>
    <div class="box-B">xxxx</div>
</div>