JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Title</title>
    <meta http-equiv="content-type" content="text/html; charset=utf-8" />
</head>
<body>
    <div id="container">
        <div class="inner">
            <div id="left">left - shall reach down until footer</div>
            <div id="right">content part...<br> shall be next to it...</div>
            <div class="clear"></div>
        </div>
        <div id="footer">footer</div>
    </div>
</body>
</html>

CSS

* {  margin: 0; }
html, body { height: 100%; }
#container {
    position: absolute; /* have no control over that container element */
    width: 400px;  height:300px;
    top: 100px;    left: 10px;
    background: red;
}
#right {
    height:     100%;
}
#footer {
    background: green;
    height: 100px;
    position:absolute;
    left:0;
    right:0;
}
#left {
    background: yellow;
    float: left; 
    width: 150px;
    height: 100%;
}
.clear {
    clear:both;
}
.inner {
    height:100%;
    margin-bottom:-100px;
}