Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<h3>A demonstration of how to access a BODY element</h3>

<p>Click the button to set the background color of the document to red.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
    var x = document.getElementsByTagName("BODY")[0];
    x.style.backgroundColor = "red";
}
</script>

</body>
</html>