<!-- BEGIN COPY / PASTE -->
        <style>
            .box {
                width: 100px;
                height: 100px;
                background-color: blue;
                transition: background-color 0.5s ease;
            }
        </style>

        <div class="box" id="box"></div>

        <script>
            const box = document.getElementById('box');
            box.addEventListener('click', () => {
                box.style.backgroundColor = 'red';
            });
        </script>
        <!-- END COPY / PASTE -->