JSFiddle - React, Tailwind, and code Playground

HTML

<div class="row-fluid">
    <div class="span4"> Hello, I'm always the same width</div>
    <div class="container-fluid">Hey, I always fill the remaining space.</div>
</div>

CSS

.row-fluid { width: 95%; margin: auto; }
.span4 { width: 200px; float:left; background: green; }
.container-fluid { margin-left: 220px; background: orange; }
div { padding: 10px;}