img {
    max-width: 100%;
    vertical-align: top;
}

.gallery {
    display: flex;
    max-width: 500px;
    position: relative;
    padding-top: 66.6666666667%;
}

@media screen and (min-width: 600px) {
    .gallery {
        padding-top: 300px;
    }
}

.gallery__img {
    position: absolute;
    width: 100%;
    top: 0;
    left: 0;
    opacity: 0;
    transition: opacity 0.3s ease-in-out;
}

.gallery__thumb {
    padding-top: 6px;
    margin: 6px;
    display: block;
}

.gallery__selector {
    position: absolute;
    opacity: 0;
    visibility: hidden;
}

.gallery__selector:checked+.gallery__img {
    opacity: 1;
}

.gallery__selector:checked~.gallery__thumb>img {
    box-shadow: 0 0 0 3px var(--c-primary);
}