.gallery {
    display: flex;
    flex-wrap: wrap;
    /* Compensate for excess margin on outer gallery flex items */
    margin: -1rem -1rem;
}

.gallery-item {
    /* Minimum width of 24rem and grow to fit available space */
    flex: 1 0 24rem;
    /* Margin value should be half of grid-gap value as margins on flex items don't collapse */
    margin: 1rem;
    box-shadow: 0.3rem 0.4rem 0.4rem rgba(0, 0, 0, 0.4);
    overflow: hidden;
}

.gallery-image {
    display: block;
    width: 60%;
    height: 60%;
    object-fit: cover;
    transition: transform 400ms ease-out;
}

    .gallery-image:hover {
        transform: scale(1.15);
    }

/*

The following rule will only run if your browser supports CSS grid.

Remove or comment-out the code block below to see how the browser will fall-back to flexbox styling. 

*/

@supports (display: grid) {
    .gallery {
        display: grid;
        grid-template-columns: repeat(auto-fit, minmax(24rem, 1fr));
        grid-gap: 2rem;
    }

    .gallery,
    .gallery-item {
        margin: 0;
    }
}