Ш.Азиз,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
#az {
position: absolute;
height: 100%;
width: 100%;
background-color: black;
left: -100%;
}
#az.anim{
animation:as 5.75s steps(40) 1 forwards;
}
@keyframes as {
0% {
left: -100%;
}
100% {
left: 0px;
}
}
</style>
<script>
document.addEventListener('DOMContentLoaded', function() {
document.querySelector('.btn').addEventListener('click',
function() {
document.querySelector('#az').classList.add("anim");
})
});
</script>
</head>
<body>
<button class="btn">btn</button>
<div id="az"></div>
</body>
</html>