JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo">This is Foo!</div>

CSS

@-webkit-keyframes fadein {
    from: { opacity: 0; }
    to: { opacity: 1; }
}

@-moz-keyframes fadein {
    from: { opacity: 0; }
    to: { opacity: 1; }
}

@keyframes fadein {
    from: { opacity: 0; }
    to: { opacity: 1; }
}

#foo {
    background-color: green;
    color: white;
    -webkit-animation: fadein 2s ease-in alternate infinite;
    -moz-animation: fadein 2s ease-in alternate infinite;
    animation: fadein 2s ease-in alternate infinite;
}