JSFiddle - React, Tailwind, and code Playground

HTML

<button id="monBouton" type="button" class="btn btn-success">connexion</button>

CSS

#monBouton {        
    -webkit-animation-name: fadeIn; /* Chrome, Safari, Opera */
    -webkit-animation-delay: 1s; /* Chrome, Safari, Opera */
    -webkit-animation-duration: 3s; /* Chrome, Safari, Opera */
    animation-name: fadeIn;
    animation-delay: 1s;
    animation-duration: 3s;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes fadeIn {
    0% {opacity: 0;}
    100% {opacity: 1;}
}
/* Standard syntax */
@keyframes fadeIn {
    0% {opacity: 0;}
    100% {opacity: 1;}
}