<div class="container"></div>
.container {
    width:  150px;
    height: 150px;
    background-color: #000;
    transition: background .5s ease-out, border-radius .5s ease-in;
    -webkit-transition: background .5s ease-out, border-radius .5s ease-in;
}
.container:hover {
    background: #e1e1e1;
    border-radius: 80px;
}

External CSS

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External JavaScript

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