.gallery  {
  --d: 30s; /* duration 25s*/
  
  display: grid;
  overflow: hidden;
  -webkit-mask: linear-gradient(90deg,#0000,#000 10% 90%,#0000);
}
.gallery > img {
  grid-area: 1/1;
  height: 150px;
 /* aspect-ratio: 1.5;*/
  object-fit: cover;
  animation: r var(--d) linear infinite;
}
.gallery > img:nth-child(2) {animation-delay: calc(1*var(--d)/-4)}
.gallery > img:nth-child(3) {animation-delay: calc(2*var(--d)/-4)}
.gallery > img:nth-child(4) {animation-delay: calc(3*var(--d)/-4)}

@keyframes r {
  75%    {transform: translate(-300%)}
  75.01% {transform: translate( 100%)}
}