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<!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>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 or Safari.</p> <script> document.getElementById("myInput").addEventListener("invalid", myFunction); function myFunction() { alert("You must fill out the form!"); } </script> </body> </html>