ples help me !!!

im so confused on what ive done wrong with this code its not working [i barely know any html stuff]

<style>

body:before {

content: " ";

height: 100vh;

width: 100vw;

display: block;

position: fixed; 

top: 0; 

left: 0; 

z-index: 100;

background-image: url('https://tenor.com/en-GB/view/medic-tf2-pigeon-victory-fabulous-gif-22434415');

background-size: cover;

background-repeat: no-repeat;

background-position:center;

animation: yourAnimation 1s ease 0s 1 normal forwards;

pointer-events: none;}

@keyframes yourAnimation { 0.0%{ opacity: 1;} 75%{ opacity: 1; } 100%{ opacity: 0;} } 

</style>