<button title="tips">button</button>

  <div class="btn">
    <span title="提示文本">鼠标移动到这</span>
    <span class="tip">提示文本</span>
  </div>

.btn {
  width: 100px;
  position: relative;
  display: inline-block;
  background-color: gold;
  padding: 5px;
  cursor: pointer;
}

.btn .tip {
  display: none;
  width: 100px;
  background-color: black;
  color: #fff;
  text-align: center;
  padding: 5px 0;
  font-size: 12px;

  /* 垂直居中 */
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  left: 118px;
  z-index: 1;
}

.btn:hover .tip {
  display: block;
}

<button class="btn" tip="a bcdef">鼠标移动到这</button>

    .btn {
      position: relative;

      display: block;
      margin: 100px auto;
      padding: 10px;
      font-size: 14px;
      background: #fff;
      color: green;
      border: 1px solid green;
      cursor: pointer;
    }
    [tip]:after {
      content: attr(tip);
      visibility: hidden;
      /* 实现垂直居中 */
      position: absolute;
      top: 50%;
      transform: translate(-5px, -50%);
      transition: all .3s;

      left: 100%;
      opacity: 0;

      /* 空白问题 */
      white-space: pre;
      font-size: 16px;
      padding: 5px 10px;
      background-color: rgba(18,26,44,0.8);
      color: #fff;
      box-shadow: 1px 1px 14px rgba(0,0,0,0.1)
    }
    [tip]:hover {
      /* ie兼容 */
      overflow: visible
    }
    .btn:hover:after {
      opacity: .8;
      transition: all .3s;
      transform: translate(5px, -50%);
      visibility: visible;
      display: block;
    }