JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="fixo"></div>
</div>

CSS

body {
    margin:0px;
    padding:0px;
}
.container {
    background: #F00;
    width:100%;
    height:300px;    
}

.fixo {
    width:100px;
    height:50px;
    background:#000;
    position:fixed;
    top:0px;
    margin:0 auto;
    left: calc(50% - 50px);
}