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                <div class="circle">レスポンシブ(画面サイズに影響されて楕円に歪まない)にて作った正円の上下左右中央に、テキストを配置する方法の紹介です。</div>


                .circle {
  max-width:  240px;/* 円のサイズ(高さにもなる) */
  border-radius: 100%;
  padding: 16px;
  background: #aaa;
  /* 以下のFlexboxで文字を中央寄せ */
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-box-align: center;
  -ms-flex-align: center;
  align-items: center; 

.circle::before {/* 擬似要素で正円を作る */
  display: block;
  content: '';
  padding-top: 100%;