JSFiddle - React, Tailwind, and code Playground

by inkc

HTML

<div class="page">
    <header>The header bar</header>
   <div class="content">the content here </div>
</div>


<div class="before-header">This is dynamically created</div>

CSS

.page {
    background:#f8f8f8;
    height:900px;
    /*margin-top: 100px;*/
}
.before-header {
    position:fixed;
    top:0;
    background:#09f;
    height:360px;
    width:100%;
    z-index:30000;
}
header {
    position:fixed;
    top:0px;
    background:#f90;
    height:30px;
    width:100%;
}

JavaScript

var finder = document.getElementsByClassName("before-header")[0];
var findrheight = finder.clientHeight;
var main = document.getElementsByTagName("header")[0];
main.style["margin-top"] = findrheight+"px";