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<!DOCTYPE html> <html> <body> <p>Click the "Try it" button to underline the H1 element:</p> <button onclick="myFunction()">Try it</button> <h1 id="myH1">This is my header</h1> <script> function myFunction() { document.getElementById("myH1").style.textDecoration = "underline"; } </script> </body> </html>