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<!DOCTYPE html> <html> <head> <style> .mystyle { width: 500px; height: 50px; border: 1px solid black; margin-bottom: 10px; } .anotherClass { background-color: coral; text-align: center; font-size: 25px; color: white; } </style> </head> <body> <p>Click the button to add an additional class to the div element.</p> <div id="myDIV" class="mystyle"> I am a DIV element </div> <button onclick="myFunction()">Try it</button> <script> function myFunction() { document.getElementById("myDIV").className += " anotherClass"; } </script> </body> </html>