title: Horizontal blank space though margin property is not set
body: <p>I ran into this problem while learning CSS. I tried to search for this but couldn't find any proper answers. Some lead me to <em>margin collapsing</em>, but it just doesn't happen to horizontal margins. </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#nav-bar {
    position: fixed;
    top: 0px;
    left: 0px;
    height: 60px;
    width: 100%;
    display: flex;
    justify-content: flex-end;
    align-items: center;
    background-color: #be3144;
  }
  
  #nav-list {
    display: flex;
    margin-right: 4rem;
  }
  
  .nav-link {
    text-decoration: none;
    color: white;
    padding: 0 1.6rem 0 1.6rem;
    height: 60px;
    weight: 100%;
    display: flex;
    align-items: center;
    font-size: 1.3rem;
  }
  
  #welcome-section {
    background-color: #3a3d40;
    height: 100vh;
    display: flex;
    flex-direction: column;
    text-align: center;
    align-items: center;
    justify-content: center;
    margin-top: 60px;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;nav id="nav-bar"&gt;
    &lt;div id="nav-list"&gt;
      &lt;a href="#welcome-section" class="nav-link"&gt;About&lt;/a&gt;
      &lt;a href="#tile" class="nav-link"&gt;Work&lt;/a&gt;
      &lt;a href="#contact" class="nav-link"&gt;Contact&lt;/a&gt;
    &lt;/div&gt;
  &lt;/nav&gt;
  
  &lt;section id="welcome-section"&gt;
    &lt;h1&gt;Hey I am Mimic&lt;/h1&gt;
    &lt;h4&gt;a web developer&lt;/h4&gt;
  &lt;/section&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>The #welcome-section below nav-bar has blank space both left and right sides, though I didn't set any margin properties. 
  Also, any advices and suggestions in styling HTML/CSS are appreciated. Thanks in advance.</p>