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<!DOCTYPE html>
<html>
<body>
<form action="/action_page.php" method="get">
  Name: <input type="text" oninvalid="alert('You must fill out the form!');" 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>
</body>
</html>