SOURCE

<div class="container">
    <div class="item" id="item"></div>
    <!-- <div class="other" id="other"></div> -->
</div>

.container {
    background: rgb(219, 212, 103);
    height: 600px;
    width: 400px;
    position: absolute;
    /* display: flex; */
    /* flex-direction: column; */
    /* position: relative; */
}

.item {
    width: 200px;
    height: 200px;
    background: rgb(255, 124, 124);
    /* margin: auto; */
    position: relative;
    margin: 50%;
}

.other {
    width: 390px;
    flex: 1;
    background: rgb(122, 252, 48);
}