JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div class="a"></div>
    <div class="b"></div>
</body>

CSS

.a {
            background-color: red;
            width: 15em;
            height: 15em;
            position: absolute;
            margin-top: -5rem;
        }

        .b {
            background-color: blue;
            width: 15em;
            height: 15em;
            margin-top: 5rem;
            animation: show-img 1s;
        }

        @keyframes show-img
        { from { opacity: 0 }
        to {opacity: 1}
        }