Default Open Accordion

Question

I am learning html/css and learned accordion menu 2 days ago. But I want to do the same thing in reverse. In other words, the menus will be open at startup, when I click on the title, they will collapse. Unfortunately I was never able to do that. Does anyone have an idea?

var acc = document.getElementsByClassName("accordion");
var i;
for (i = 0; i < acc.length; i++) {
  acc[i].addEventListener("click", function() {
    this.classList.toggle("active");
    var panel = this.nextElementSibling;
    if (panel.style.maxHeight) {
      panel.style.maxHeight = null
    } else {
      panel.style.maxHeight = panel.scrollHeight + "px";
    }
  });
}

.accordion {
  background-color: #FFFFFF;
  cursor: pointer;
  padding: 18px;
  width: 100%;
  border: none;
  text-align: left;
  font-size: 15px;
}

.accordion:after {
  content: "\002B";
  /* plus sign */
  font-weight: bold;
  float: right;
  margin-left: 5px;
}

.active:after {
  content: "\2212";
}


/* minus sign */

.panel {
  padding: 0 18px;
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.2s ease-out;
}

<table width="750">
  <td style="width: 100%;">

    <button class="accordion">Header 1</button>
    <div class="panel">
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor</p>
    </div>

    <button class="accordion">Header 2</button>
    <div class="panel">
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor</p>
    </div>

    <button class="accordion">Header 3</button>
    <div class="panel">
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor</p>
    </div>

  </td>
</table>

Answer 1

Simple add manually active on accordion button and style="max-height: 50px;" to panel

var acc = document.getElementsByClassName("accordion");
var i;
for (i = 0; i < acc.length; i++) {
  acc[i].addEventListener("click", function() {
    this.classList.toggle("active");
    var panel = this.nextElementSibling;
    if (panel.style.maxHeight) {
      panel.style.maxHeight = null
    } else {
      panel.style.maxHeight = panel.scrollHeight + "px";
    }
  });
}

.accordion {
  background-color: #FFFFFF;
  cursor: pointer;
  padding: 18px;
  width: 100%;
  border: none;
  text-align: left;
  font-size: 15px;
}

.accordion:after {
  content: "\002B";
  /* plus sign */
  font-weight: bold;
  float: right;
  margin-left: 5px;
}

.active:after {
  content: "\2212";
}


/* minus sign */

.panel {
  padding: 0 18px;
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.2s ease-out;
}

<table width="750">
  <td style="width: 100%;">

    <button class="accordion active">Header 1</button>
    <div class="panel" style="max-height: 50px;">
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor</p>
    </div>

    <button class="accordion active">Header 2</button>
    <div class="panel" style="max-height: 50px;">
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor</p>
    </div>

    <button class="accordion active">Header 3</button>
    <div class="panel" style="max-height: 50px;">
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor</p>
    </div>

  </td>
</table>