CSSツールチップの作り方｜補足説明を吹き出しで出すコピペ集

<p>
  この処理は
  <span class="tip" data-tip="ブラウザが表示を組み立て直すこと。頻発すると重くなります。">
    リフロー
  </span>
  を発生させます。
</p>

.tip {
  position: relative;         /* 吹き出しの基準 */
  border-bottom: 1px dotted #6b7280;
  cursor: help;
}

.tip::after {
  content: attr(data-tip);    /* 属性の値をそのまま表示 */
  position: absolute;
  bottom: calc(100% + 10px);  /* 要素の上に出す */
  left: 50%;
  transform: translateX(-50%);
  width: max-content;
  max-width: 260px;
  padding: 10px 14px;
  border-radius: 8px;
  background-color: #1f2937;
  color: #fff;
  font-size: 0.85rem;
  line-height: 1.6;
  text-align: left;
  opacity: 0;
  visibility: hidden;
  transition: opacity .2s, visibility .2s;
  z-index: 50;
}

.tip:hover::after,
.tip:focus-visible::after {
  opacity: 1;
  visibility: visible;
}

/* 上（既定） */
.tip.-top::after {
  bottom: calc(100% + 10px);
  left: 50%;
  transform: translateX(-50%);
}

/* 下 */
.tip.-bottom::after {
  top: calc(100% + 10px);
  bottom: auto;
  left: 50%;
  transform: translateX(-50%);
}

/* 左 */
.tip.-left::after {
  right: calc(100% + 10px);
  left: auto;
  bottom: auto;
  top: 50%;
  transform: translateY(-50%);
}

/* 右 */
.tip.-right::after {
  left: calc(100% + 10px);
  bottom: auto;
  top: 50%;
  transform: translateY(-50%);
}

.tip::before {
  content: "";
  position: absolute;
  bottom: calc(100% + 4px);
  left: 50%;
  transform: translateX(-50%);
  border: 6px solid transparent;
  border-top-color: #1f2937;   /* 本体と同じ色 */
  opacity: 0;
  visibility: hidden;
  transition: opacity .2s, visibility .2s;
  z-index: 51;
}

.tip:hover::before,
.tip:focus-visible::before {
  opacity: 1;
  visibility: visible;
}

/* 下方向のときは三角も反転 */
.tip.-bottom::before {
  bottom: auto;
  top: calc(100% + 4px);
  border-top-color: transparent;
  border-bottom-color: #1f2937;
}

.tip::after {
  transform: translateX(-50%) translateY(6px);
  transition: opacity .18s ease, transform .18s ease, visibility .18s;
}

.tip:hover::after,
.tip:focus-visible::after {
  transform: translateX(-50%) translateY(0);
}

/* 表示までの待ち時間を入れると誤爆が減る */
.tip::after,
.tip::before {
  transition-delay: .25s;
}

@media (prefers-reduced-motion: reduce) {
  .tip::after,
  .tip::before {
    transition: none;
  }
}

.tip.-wide::after {
  max-width: 320px;
  white-space: normal;      /* 折り返しを許可 */
  line-height: 1.7;
  padding: 12px 16px;
}

/* 改行を入れたい場合はdata属性に改行文字を含める */
.tip.-pre::after {
  white-space: pre-line;
}

<span class="tip -pre" data-tip="1行目の説明です。
2行目はこう書きます。">用語</span>

/* 左端用：左を基準にする */
.tip.-align-start::after {
  left: 0;
  transform: none;
}

/* 右端用：右を基準にする */
.tip.-align-end::after {
  left: auto;
  right: 0;
  transform: none;
}

/* 親のはみ出し切り取りを避ける */
.table-wrap {
  overflow-x: auto;         /* 表の横スクロールは必要 */
}

.table-wrap .tip::after {
  position: fixed;          /* 切り取られたくない場合の回避策 */
}

/* 1. キーボードでも開く */
.tip {
  /* HTML側で tabindex="0" を付けておく */
}

.tip:focus-visible {
  outline: 2px solid #0ea5e9;
  outline-offset: 2px;
}

.tip:focus-visible::after,
.tip:focus-visible::before {
  opacity: 1;
  visibility: visible;
}

/* 2. タッチ端末ではタップで開く（:focusを利用） */
@media (hover: none) {
  .tip:focus::after,
  .tip:focus::before {
    opacity: 1;
    visibility: visible;
  }
}

<span class="tip" tabindex="0" role="note"
      data-tip="ブラウザが表示を組み立て直すこと。">リフロー</span>

.tip {
  position: relative;
  border-bottom: 1px dotted #6b7280;
  cursor: help;
}

.tip::after,
.tip::before {
  opacity: 0;
  visibility: hidden;
  transition: opacity .18s ease, transform .18s ease, visibility .18s;
  transition-delay: .25s;
  pointer-events: none;
}

.tip::after {
  content: attr(data-tip);
  position: absolute;
  bottom: calc(100% + 10px);
  left: 50%;
  transform: translateX(-50%) translateY(6px);
  width: max-content;
  max-width: 260px;
  padding: 10px 14px;
  border-radius: 8px;
  background-color: #1f2937;
  color: #fff;
  font-size: 0.85rem;
  line-height: 1.6;
  text-align: left;
  white-space: normal;
  z-index: 50;
}

.tip::before {
  content: "";
  position: absolute;
  bottom: calc(100% + 4px);
  left: 50%;
  transform: translateX(-50%);
  border: 6px solid transparent;
  border-top-color: #1f2937;
  z-index: 51;
}

.tip:hover::after,
.tip:hover::before,
.tip:focus-visible::after,
.tip:focus-visible::before {
  opacity: 1;
  visibility: visible;
}

.tip:hover::after,
.tip:focus-visible::after {
  transform: translateX(-50%) translateY(0);
}

.tip:focus-visible {
  outline: 2px solid #0ea5e9;
  outline-offset: 2px;
}

@media (hover: none) {
  .tip:focus::after,
  .tip:focus::before {
    opacity: 1;
    visibility: visible;
  }
}

@media (prefers-reduced-motion: reduce) {
  .tip::after,
  .tip::before {
    transition: none;
  }
}

.tip.-light::after {
  background-color: #fff;
  color: #1f2937;
  border: 1px solid #e5e7eb;
  box-shadow: 0 8px 24px rgba(15, 23, 42, .12);
}

.tip.-light::before {
  border-top-color: #fff;
}

.tip.-warn::after {
  background-color: #b91c1c;
  color: #fff;
  font-weight: 700;
}

.tip.-warn::before {
  border-top-color: #b91c1c;
}

.tip.-icon {
  border-bottom: none;
}

.tip.-icon::first-line {
  color: inherit;
}

.tip-mark {
  display: inline-grid;
  place-items: center;
  width: 18px;
  height: 18px;
  margin-left: 4px;
  border-radius: 50%;
  background-color: #94a3b8;
  color: #fff;
  font-size: 12px;
  font-weight: 700;
  vertical-align: middle;
}

<span class="tip -icon" tabindex="0" data-tip="税抜価格です。">
  月額料金<span class="tip-mark">?</span>
</span>

.tip.-lg::after {
  padding: 14px 18px;
  border-radius: 14px;
  font-size: 0.95rem;
  max-width: 340px;
}

.tip.-instant::after,
.tip.-instant::before {
  transition-delay: 0s;
}

/* 基本形を1クラスにまとめておき、必要な時だけ方向クラスを足す */
.tip { position: relative; cursor: help; }
.tip.-bottom::after { top: calc(100% + 10px); bottom: auto; }
.tip.-right::after  { left: calc(100% + 10px); top: 50%; bottom: auto;
                      transform: translateY(-50%); }