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<!DOCTYPE html>
<html>
<body>

<button type="button" id="myBtn" onclick="myFunction()">Change the width of this button</button>

<script>
function myFunction() {
document.getElementById("myBtn").style.width = "300px";
}
</script>

</body>
</html>

<!DOCTYPE html>
<html>
<body>

<button type="button" id="myBtn" onclick="myFunction()">Change the width of this button</button>

<script>
function myFunction() {
    document.getElementById("myBtn").style.width = "300px";
}
</script>

</body>
</html>