CSS

.container {
  width: 960px;
  margin: 0 auto;
}

.col {
  float: left;
  padding-left: 20px
  box-sizing: border-box;
}

.col:first-child {
  padding-left: 0;
}

.main-col {
  width: 600px;
}

.sidebar {
  width: 360px;
}

/**
 * For modern browsers
 * 1. The space content is one way to avoid an Opera bug when the
 *    contenteditable attribute is included anywhere else in the document.
 *    Otherwise it causes space to appear at the top and bottom of elements
 *    that are clearfixed.
 * 2. The use of `table` rather than `block` is only necessary if using
 *    `:before` to contain the top-margins of child elements.
 */
.cf:before,
.cf:after {
    content: " "; /* 1 */
    display: table; /* 2 */
}

.cf:after {
    clear: both;
}

/**
 * For IE 6/7 only
 * Include this rule to trigger hasLayout and contain floats.
 */
.cf {
    *zoom: 1;
}

HTML

<div class="container">
  <header class="site-header">
  </header>
  <section class="main">
    <div class="cf">
      <div class="col sidebar">
      </div>
      <div class="col main-col">
      </div>
    </div>
  </section>
  <footer>
  </footer>
</div>