<p>Вибери потрібне значення(пікселі) для <code>flex-basis</code> (для лівої навігації):</p>

<input type="number" value="200" id="field"></input>

<p id="text"></p>

<hr>

<div class="layout">
  <header>
    <h4>Simple website</h4>
  </header>
  <main>
    <nav id="nav">
      <pre><code>flex: <strong id="newval">200</strong>px;</code></pre>
    </nav>
    <section id="wrapper">
      <pre><code>flex-basis: auto;</code>
    </section>
  </main>
  <footer>
    &copy; 2017
  </footer>
</div>
* {
  box-sizing: border-box;
  padding: 0;
}

body {
  text-align: center;
  font-family: momospace;
  font-size: 17px;
  line-height: 1.5;
  color: #222;
}

#field {
  padding: 8px 16px;
  font-size: 19px;
}

header {
  line-height: 75px;
  text-align: center;
  text-transform: uppercase;
  background-color: #808080;
  color: #f9f9f9;
}

main {
  display: flex;
}

nav {
  background-color: yellow;
  flex-basis: 200px;
}

nav a {
  display: block;
  margin-bottom: 16px;
  text-align: left;
  text-decoration: none;
  color: #151515;
}

#wrapper {
  flex-basis: auto;
  background-color: #eee;
}

footer {
  line-height: 50px;
  background-color: #000;
  color: #f1f1f1;
  margin-top: 16px;
}
var nav = document.getElementById('nav');
var newval = document.getElementById('newval');

document.getElementById('field').onchange = function() {
  nav.style.flexBasis = this.value + 'px';
  newval.innerHTML = this.value;
}

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