JSFiddle - React, Tailwind, and code Playground

by Alarr

HTML

<div class="body">
    <div class="topBlock"></div>
    <div class="bottomBlock"></div>    
</div>

CSS

.body {
    width: 100%;
    height: 200px;
    outline: 1px solid red;
}

.topBlock {
    width: 200px;
    height: 50px;
    background: green;
    position: fixed;
    left: 50%;
    margin-left: -100px; /* отнять половину ширины блока */
}
.bottomBlock {
    width: 200px;
    height: 50px;
    background: blue;
    margin: 0 auto;
    position: relative;
    top: 50px;
}