Remove whitespace above div in react app

Most browsers (eg. Chrome) come with a default set of rules (user agent stylesheet) and set rules like margin in ul's, so you likely have a margin-top (-webkit-margin-before: 1em;) set to your ul.

Set margin-top: 0 on the ul will remove the space:

ul {
  margin-top: 0;
}

I've set the margin for ul to zero (and included padding to force a default reset). Let me know if this meets your requirements.

You may want to have a look at tools like normalize.css for future use.

body {background-color: red;}
body, ul {
  margin: 0;
  padding: 0;
}

.container {
  background-color: #bec0c4;
  height: 50px;
  width: 100%;
}

.container ul {
  display: flex;
  flex-direction: row;
  font-size: 20px;
  justify-content: space-between;
  align-items: center;
  list-style-type: none;
  width: 90%;
  height: 100%;
}
<div class="container">
  <ul>
    <div class="links">
      <li>
        <a>Quizzes</a>
      </li>
    </div>
    <div class="links">
      <li>
        <a>Categories</a>
      </li>
    </div>
    <div class="links">
      <li>
        <a>Create</a>
      </li>
    </div>
  </ul>
</div>