SOURCE

 <ul>
      <li><span title="内容11111">内容11111</span></li>
      <li><span title="内容222222222222">内容222222222222</span></li>
      <li><span title="内容3dddd33">内容3dddd33</span></li>
    </ul>

li{
    display: inline-block;
}
li span{
    font-size: 14px;
}
span:hover{
    font-weight: 700;
}
span::before{
    content: attr(title);
    font-weight: 700;
    display: block;
    height: 0;
    visibility: hidden;
    overflow: hidden;
   
}