html – Hover Activated Accordion + Clean transitioning between states

/* The Magic */
.accordion:hover .accordion-item:hover .accordion-item-content,
.accordion .accordion-item--default .accordion-item-content {
    peak: 2em;
}

.accordion-item-content,
.accordion:hover .accordion-item-content {
    peak: 0;
    overflow: hidden;
    transition: peak .25s;
}

/* Aesthetic */
physique {
  background: #222;
}

.accordion {
    padding: 0;
    margin: 0 auto;
    width: 100%;
    font-family: "PT Sans" sans-serif
}

.accordion .accordion-item {
  background-image: linear-gradient(90deg, #eee, #f5f5f5, #eee);
  border-bottom: 1px strong #666;
  padding: 1em;
  shade: #eee;
}

.accordion h1 {
  margin: 0;
}

.accordion .accordion-item:nth-child(1) {
  background-color: hsl(200, 80%, 60%);
  background-image: linear-gradient(-90deg, hsl(200, 80%, 60%), hsl(200, 80%, 45%) 2em, hsl(200, 80%, 60%));

}

.accordion .accordion-item:nth-child(2) {
  background-color: hsl(280, 60%, 45%);
  background-image: linear-gradient(-90deg, hsl(280, 80%, 60%), hsl(280, 60%, 45%) 2em, hsl(280, 80%, 60%));
}

.accordion .accordion-item:nth-child(3) {
  background-color: hsl(40, 80%, 40%);
  background-image: linear-gradient(-90deg, hsl(40, 80%, 60%), hsl(40, 80%, 45%) 2em, hsl(40, 80%, 60%));
}

.accordion .accordion-item:nth-child(4) {
  background-color: hsl(80, 40%, 40%);
  background-image: linear-gradient(-90deg, hsl(80, 80%, 40%), hsl(80, 80%, 35%) 2em, hsl(80, 80%, 40%));
}

<div class="accordion">
    <part class="accordion-item">
        <h1>Part 1</h1>
        <div class="accordion-item-content">
          <p font-size="20px">Content material for part 1</p>
        </div>
    </part>
    <part class="accordion-item">
        <h1>Part 2</h1>
        <div class="accordion-item-content">
          <p>Content material for part 2</p>
        </div>
    </part>
    <part  class="accordion-item">
        <h1>Part 3</h1>
        <div class="accordion-item-content">
          <p>Content material for part 3</p>
          <p>That is my default state.</p>
        </div>
    </part>
    <part class="accordion-item">
        <h1>Part 4</h1>
        <div class="accordion-item-content">
          <p>Content material for part 4</p>
        </div>
    </part>
</div>