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HTML 

              
                <section class="faq ">
  <div class="inner">
    <h2 class="title">よくある質問</h2>
    <div class="faq__contents">
      <details class="faq__details js-details">
        <summary class="faq__summary js-summary">
          質問1<span class="arrow-icon"></span>
        </summary>
        <div class="faq__content js-content">
          <p class="faq__text">回答1テキストの部分です。回答1テキストの部分です。回答1テキストの部分です。回答1テキストの部分です。回答1テキストの部分です。回答1テキストの部分です。</p>
        </div>
      </details>
      <details class="faq__details js-details">
        <summary class="faq__summary js-summary">
          質問2<span class="arrow-icon"></span>
        </summary>
        <div class="faq__content js-content">
          <p class="faq__text"> 回答2テキストの部分です。回答2テキストの部分です。回答2テキストの部分です。回答2テキストの部分です。回答2テキストの部分です。回答2テキストの部分です。</p>
        </div>
      </details>
      <details class="faq__details js-details">
        <summary class="faq__summary js-summary">
          質問3<span class="arrow-icon"></span>
        </summary>
        <div class="faq__content js-content">
          <p class="faq__text">回答3テキストの部分です。回答3テキストの部分です。回答3テキストの部分です。回答3テキストの部分です。回答3テキストの部分です。回答3テキストの部分です。</p>
        </div>
      </details>
    </div>
  </div>
</section>
!

CSS 

              
                * {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  line-height: 1.6;
}

summary {
  display: block;
}

summary::-webkit-details-marker {
  display: none;
}
.faq {
  margin-top: 80px;
  margin-bottom: 80px;
}
.inner {
  max-width: 900px;
  padding: 0 4%;
  margin:0 auto;
}
.faq__contents {
  margin-top: 24px;
}
.faq__details {
  padding: 16px;
}
.faq__summary {
  cursor: pointer;
  position: relative;
  display: flex;
  justify-content: space-between;
  padding-left: 32px;
}

.faq__summary::before,
.faq__text::before {
  position: absolute;
  left: 0;
  height: 24px;
  width: 24px;
  border-radius: 50px;
  text-align: center;
  display: block;
}

.faq__summary::before {
  content: "Q";
  background-color: #2f3135;
  color: #ffffff;
  top: 0;
}

.faq__text::before {
  content: "A";
  background-color: #fd952e;
  color: #ffffff;
  top: 50%;
  transform: translateY(-50%);
}

/* 矢印アイコン */
.arrow-icon {
  display: inline-block;
  position: relative;
  transition: transform 0.4s ease;
  height: 16px;
  width: 16px;
  transform: rotate(135deg);
}

.arrow-icon::before {
  content: "";
  width: 10px;
  height: 10px;
  border-top: solid 2px #2f3135;
  border-right: solid 2px #2f3135;
  position: absolute;
  top: 50%;
  right: 50%;
}

/* アクティブな状態の矢印アイコン */
.arrow-icon.active {
  transform: rotate(315deg);
}
.faq__text {
  padding: 16px 32px;
  position: relative;
}

.faq__content {
  overflow: hidden;
}
!

JS 

              
                document.querySelectorAll(".faq__details").forEach((details) => {
  const arrowIcon = details.querySelector(".arrow-icon");
  details.querySelector(".faq__summary").addEventListener("click", () => {
    arrowIcon.classList.toggle("active");
  });
});
!
999px

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