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Result Size: 625 x 571
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<!DOCTYPE html>
<html>
<body>
<form action="https://www.w3schools.com/action_page.php" method="get">
  Name: <input type="text" id="myInput" name="fname" required>
  <input type="submit" value="Submit">
</form>
<p>If you click submit, without filling out the text field,
an alert message will occur.</p>
<p><strong>Note:</strong> The oninvalid event is not supported in Internet Explorer 9 and earlier.</p>
<script>
document.getElementById("myInput").addEventListener("invalid", myFunction);
function myFunction() {
  alert("You must fill out the form!");
}
</script>
</body>
<!-- Mirrored from www.w3schools.com/jsref/tryit.asp?filename=tryjsref_oninvalid_addeventlistener by HTTrack Website Copier/3.x [XR&CO'2014], Mon, 27 Jan 2020 03:02:56 GMT -->
</html>
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