JSFiddle - React, Tailwind, and code Playground

HTML

<div id="expand-box">
   <div id="expand-box-header">
      <span style="float: left;">Top left header</span> 
      <span style="float: right;">Top right header</span>
       <div class="clear"></div>
   </div>
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</div>

CSS

#expand-box
{
    width: 100%;
    padding: 0;
    border: 2px solid #BBB;
    margin: 7px 0 0 0;
}

#expand-box-header
{
    background-color: #BBB;
    margin: 0;
    color: #FFF;
    padding: 0 0 3px 2px;
}
.clear{
    clear:both;
}