获取默认打开的CSS选项卡

<div class="tab">
  <button class="tablinks active" onclick="openTab(event, 'tab1')">选项卡1</button>
  <button class="tablinks" onclick="openTab(event, 'tab2')">选项卡2</button>
  <button class="tablinks" onclick="openTab(event, 'tab3')">选项卡3</button>
</div>

<div id="tab1" class="tabcontent">
  <h3>选项卡1 内容</h3>
  <p>这是选项卡1的内容。</p>
</div>

<div id="tab2" class="tabcontent">
  <h3>选项卡2 内容</h3>
  <p>这是选项卡2的内容。</p>
</div>

<div id="tab3" class="tabcontent">
  <h3>选项卡3 内容</h3>
  <p>这是选项卡3的内容。</p>
</div>

.tabcontent {
  display: none;
}

.tablinks {
  background-color: #ccc;
  color: #000;
  padding: 10px 20px;
  border: none;
  cursor: pointer;
}

.tablinks.active {
  background-color: #f1f1f1;
}

/* 默认打开第一个选项卡 */
#tab1 {
  display: block;
}

function openTab(evt, tabName) {
  var i, tabcontent, tablinks;
  
  tabcontent = document.getElementsByClassName("tabcontent");
  for (i = 0; i < tabcontent.length; i++) {
    tabcontent[i].style.display = "none";
  }
  
  tablinks = document.getElementsByClassName("tablinks");
  for (i = 0; i < tablinks.length; i++) {
    tablinks[i].className = tablinks[i].className.replace(" active", "");
  }
  
  document.getElementById(tabName).style.display = "block";
  evt.currentTarget.className += " active";
}

// 默认打开第一个选项卡
document.getElementById("tab1").style.display = "block";
document.getElementsByClassName("tablinks")[0].className += " active";