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

<input type="number" min="0" value="1" id="field"></input>

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

<hr>

<div class="layout">
  <header>
    <h4>Simple website</h4>
  </header>
  <div class="box">
    <pre><code>width: 750px;</code></pre>
    <main>
    <nav id="nav">
      <pre><code>flex-shrink: <strong id="newval">1</strong>;<br>flex-basis: 350px;</code></pre>
    </nav>
    <section id="wrapper">
      <pre><code>flex-shrink: 2;<br>flex-basis: 350px;</code>
    </section>
  </main>
 </div>
  <footer>
    &copy; 2017
  </footer>
</div>
* {
  box-sizing: border-box;
  padding: 0;
}

body {
  text-align: center;
  font-family: momospace;
  font-size: 15px;
  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;
}

.box {
  width: 750px;
  border: 4px dashed red;
}

main {
  display: flex;
  width: 500px;
}

nav {
  background-color: yellow;
  flex-shrink: 1;
flex-basis: 350px;
}

#wrapper {
  background-color: #eee;
  flex-shrink: 2;
flex-basis: 350px;
}

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.flexShrink = this.value;
  newval.innerHTML = this.value;
}

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