Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<form action="demo_form.asp" method="get">
  Name: <input type="text" id="myInput" name="fname" required>
  <input type="submit" value="Submit">
</form>

<p><strong>Note:</strong> The oninvalid event is not supported in Internet Explorer 9 and earlier or Safari.</p>

<script>
document.getElementById("myInput").oninvalid = function() {myFunction()};

function myFunction() {
    alert("You must fill out the form!");
}
</script>

</body>
</html>