JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container" style="background-color: #f2f2f2">
    <div id="content">Here is the content, the height of this div will vary depending on the content.
        <br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
    </div>
    <div id="sidebar">I want this sidebar to extend, ALL the way down, to match the #content divs height (whatever it is)</div>
    <div style="clear:both"></div>
</div>

CSS

#content
{
    border:1px solid black;
    float: left;
    width: 80%;
}

#sidebar
{
    width: 19%;
    border:1px solid red;
    float: left; 
    background-color: #ddd;
}