Flex Example

HTML

<div class="container">
    <div class="one"></div>
    <div class="two"></div>
</div>

CSS

body {
    margin: 0;
    padding: 0;
}

.container {
    /* BE 100% OF PARENT? */
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}

.one {
    /* TAKE UP ALL REST OF HEIGHT? */
    background: red;
    flex: 1;
}

.two {
    height: 60px;
    background: blue;
}