<style>
.container {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    background-color: lightcyan;
    border: 10px solid red;
}

.item {
    padding: 20px;
    background-color: lightgreen;
    border: 10px solid papayawhip;
}
</style><div class="container"><div class="item">1</div></div>