:root {
  --transition-time: 600ms;
}

.accordion,
.accordion__item {
  display: flex;
  flex-direction: column;
  justify-content: flex-start;
  align-items: stretch;
}

.accordion {
  border: 1px solid #555;
  padding: 5px;
  max-width: 100%;
  width: 600px;
  height: 400px;
  gap: 5px;
}

.accordion .accordion__item {
  min-width: 0;
  min-height: 0;

  transition: flex var(--transition-time);
  flex: 0 0 auto;
}

.accordion .accordion__item.accordion__item--active {
  flex: 1 0 0;
}

.accordion .accordion__item .accordion__item__head {
  padding: 5px;
  cursor: pointer;
  user-select: none;

  transition: background var(--transition-time), border var(--transition-time);
  background: #ddd;
  border: 1px solid #999;
}

.accordion .accordion__item .accordion__item__head:hover {
  background: #bdf;
}

.accordion .accordion__item.accordion__item--active .accordion__item__head {
  background: #8af;
  border: 1px solid #57f;
}

.accordion .accordion__item .accordion__item__body {
  overflow: hidden;
  flex: 1 0 0;

  transition: opacity var(--transition-time), padding var(--transition-time),
    border var(--transition-time);
  opacity: 0;
  padding: 0 10px;
  border: 1px solid #00000000;
}

.accordion .accordion__item.accordion__item--active .accordion__item__body {
  opacity: 1;
  padding: 10px;
  border: 1px solid #999;
}