position: fixed;
top: 0;
left: 0;
z-index: 100;
background-image: url('https://i.pinimg.com/originals/bc/a3/59/bca359d4f29bc3393f3b38aee53aed4b.gif');
background-size: cover;
background-repeat: no-repeat;
background-position:center;
animation: yourAnimation 3s ease 0s 1 normal forwards;
pointer-events: none;
}
@keyframes yourAnimation { 0.0%{ opacity: 1;} 75%{ opacity: 1; } 100%{ opacity: 0;} }