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                <!-- ボタンをマウスオーバーするとダイアログが表示されます -->
<div class="block center">
  <div class="block__button">ログイン</div>
  <div class="block__dialog">ダイアログ</div>


                .block {
  position: relative;
  z-index: 0;
.block__button {
  display: inline-block;
  padding: .5em;
  border-radius: 4px;
  background-color: black;
  color: white;
  cursor: pointer;

.block__dialog {
  opacity: 0;
  visibility: hidden;
  position: absolute;
  z-index: 1;
  top: 50px;
  left: 0px;
  padding: .5em;
  transform: translate(0,-5px);
  border: 1px solid silver;
  border-radius: 4px;
  background-color: whitesmoke;
  box-shadow: 0 0 8px 0 rgba(0,0,0,.2);
  transition: opacity .2s linear 0s,
              visibility .2s linear 0s,
              transform .2s ease-in-out 0s;
  &:after {
    content: '';
    position: absolute;
    z-index: 1;
    left: 10px;
    width: 0;
    height: 0;
    border: 8px solid transparent;
  &:before {
    border-bottom: 8px solid silver;
    top: -17px;
  &:after {
    border-bottom: 8px solid whitesmoke;
    top: -16px;

.block__button:hover + .block__dialog {
  opacity: 1;
  visibility: visible;
  transform: translate(0,0);