JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

@-webkit-keyframes fadein {
    0%   { opacity: 0; }
    100% { opacity: 1; }
}

@-moz-keyframes fadein {
    0%   { opacity: 0; }
    100% { opacity: 1; }
}

@keyframes fadein {
    0%   { opacity: 0; }
    100% { 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;
}