<div class="container">
  <div class="item">Item 1</div>
  <div class="item">Item 2</div>
  <div class="item">Item 3</div>
  <hr class="delimiter">
  <div class="item">Item 4</div>
  <div class="item">Item 5</div>
  <div class="item">Item 6</div>
</div>
<form id="changer">
  <label>
    <input type="radio" name="type" value="last-child">
    .item:last-child
  </label>
  <label>
    <input type="radio" name="type" value="not-last-child">
    .item:not(:last-child)
  </label>
  <label>
    <input type="radio" name="type" value="last-of-type">
    .item:last-of-type
  </label>
  <label>
    <input type="radio" name="type" value="not-last-of-type">
    .item:not(:last-of-type)
  </label>
  <label>
    <input type="radio" name="type" value="plus" checked>
    .item + .item
    </label>
</form>

body {
  margin: 0;
  font: 16px 'Segoe UI', Arial, Helvetica, sans-serif;
}

.container {
  padding: 20px;
  display: flex;
  flex-direction: column;
  background: #ecfecf;
}

/* last-child */
.container[data-type="last-child"] .item {
  margin-bottom: 20px;
}

.container[data-type="last-child"] .item:last-child {
  margin-bottom: 0;
}

/* not-last-child */
.container[data-type="not-last-child"] .item:not(:last-child) {
  margin-bottom: 20px;
}

/* last-of-type */
.container[data-type="last-of-type"] .item {
  margin-bottom: 20px;
}

.container[data-type="last-of-type"] .item:last-of-type {
  margin-bottom: 0;
}

/* not-last-of-type */
.container[data-type="not-last-of-type"] .item:not(:last-of-type) {
  margin-bottom: 20px;
}

/* plus */
.container[data-type="plus"] .item + .item {
  margin-top: 20px;
}

.item {
  padding: 10px;
  border-radius: 4px;
  box-shadow: 0 4px 8px -2px rgba(0, 0, 0, 0.25);
  background: #ffffff;
}

.delimiter {
  height: 1px;
  margin: 20px 0;
  border: none;
  background: linear-gradient(to right, transparent, #222222, transparent);
}

#changer {
  position: fixed;
  top: 0;
  right: 0;
  padding: 5px;
  display: flex;
  flex-direction: column;
  font-family: monospace;
  background: rgba(0, 0, 0, 0.6);
  color: #ffffff;
}

#changer label {
  display: flex;
  align-items: center;
}

#changer input {
  margin: 0;
}

const container = document.querySelector('.container');
const changer = document.querySelector('#changer');

const update = () => {
  const form = new FormData(changer);
  
  container.dataset.type = form.get('type');
};

changer.addEventListener('change', update);

update();