JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
<div id="e">
</div>
</div>

CSS

.wrapper {
    -webkit-animation: fadeIn .5s ease-out 0 1;
}

#e { 
    width: 100px;
    height: 100px;
    background-color: #000;
}
#e:hover { 
    -webkit-animation: pulse 100ms ease-in-out 0s infinite alternate; } /* Works fine, pulses on hover */

@-webkit-keyframes fadeIn {
        0%   { opacity: 0; }
        100% { opacity: 0.5;}
}

@-webkit-keyframes pulse {
        0%   { background-color: #000; }
        100% { background-color: #c00; }
}