BFE.devでのAI解答例 - CSS 質問
1. 垂直方向に中央揃え

.outer { display: flex; justify-content: center; align-items: center; width: 100%; height: 100%; background-color: #efefef; }

.inner { width: 100px; height: 100px; background-color: #f44336; }