How can I center a child element both vertically and horizontally in a flex container?

<!-- BEGIN COPY / PASTE -->
    <div class="flex-container">
      <div class="child">Centered</div>
    </div>

    <style>
      .flex-container {
        display: flex;
        justify-content: center; /* Centers horizontally */
        align-items: center;    /* Centers vertically */
        height: 200px;          /* Example height */
        border: 1px solid #ccc; /* Optional styling */
      }
      .child {
        padding: 10px;
        background-color: #f0f0f0; /* Optional styling */
      }
    </style>
    <!-- END COPY / PASTE -->