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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>JS Add or Replace CSS Classes Demo</title> <style> .highlight { background: yellow; } </style> </head> <body> <div id="info" class="disabled">Something very important!</div> <script> // Selecting element var elem = document.getElementById("info"); elem.className = "note"; // Add or replace all classes with note class elem.className += " highlight"; // Add a new class highlight </script> </body> </html>