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<head>
<style>
#square {
width: 50px;
height: 50px;
background-color: green;
position: absolute;
left: 0;
top: 0;
}
</style>
</head>
<body>
<div id="square"></div>
<script>
const square = document.getElementById('square');
let pos = 0;

function moveRight() {
if (pos < window.innerWidth - 50) {
pos += 2;
square.style.left = pos + 'px';
requestAnimationFrame(moveRight);
} else {
pos = 0;
requestAnimationFrame(moveDown);
}
}

function moveDown() {
if (pos < window.innerHeight - 50) {
pos += 2;
square.style.top = pos + 'px';
requestAnimationFrame(moveDown);
}
}