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<!DOCTYPE html>
<html>
<body style="background:#f3f3f3 url('img_tree.png');background-repeat:repeat-y;">

<h1>Hello World!</h1>

<button type="button" onclick="myFunction()">Get background-repeat value</button>

<script>
function myFunction() {
    alert(document.body.style.backgroundRepeat);
}
</script>

</body>
</html>