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<html><head> <meta name="viewport" content="width=device-width, initial-scale=1"> <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.2/dist/css/bootstrap.min.css"> <script src="https://cdn.jsdelivr.net/npm/jquery@3.7.1/dist/jquery.slim.min.js"></script> <script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.1/dist/umd/popper.min.js"></script> <script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.2/dist/js/bootstrap.bundle.min.js"></script> <style> .collapsing { background-color: yellow; }</style></head><body><div class="container"> <h2>Collapsing</h2> <button type="button" class="btn btn-primary collapsed" data-toggle="collapse" data-target="#demo">Simple collapsible</button> <div id="demo" class="collapse"> The .collapsing class is added when the transition starts, and removed when it is finished. In this example, the collapsible content gets a yellow background for those seconds it takes to show the content. The background color is removed when the transition has ended. </div></div> </body></html>