[Solved] How is max width being set on this site? [closed]

HTML

<div class="container">
    <div class="inner-w">
        Stuff in your inner column
    </div>
</div>

CSS

body {
    margin: 0;
}

.container {
    background-color: #f06;
}

.container .inner-w {
    max-width: 50em; /* or 400px etc */
    margin-right: auto;
    margin-left: auto;
}

a jsFiddle that also shows this column width – and how it can be different in each sections etc.