div 水平垂直居中

<div class="parent">
  <div class="child"></div>
</div>

div.parent {
    display: flex;
    justify-content: center;
    align-items: center;
}

div.parent {
    position: relative; 
}
div.child {
    position: absolute; 
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);  
}
/* 或者 */
div.child {
    width: 50px;
    height: 10px;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-left: -25px;
    margin-top: -5px;
}
/* 或 */
div.child {
    width: 50px;
    height: 10px;
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
    

div.parent {
    display: grid;
}
div.child {
    justify-self: center;
    align-self: center;
}

div.parent {
    font-size: 0;
    text-align: center;
    &::before {
        content: "";
        display: inline-block;
        width: 0;
        height: 100%;
        vertical-align: middle;
    }
}
div.child{
  display: inline-block;
  vertical-align: middle;
}

div.parent {
display: table;
}
div.child {
display: table-cell
vertical-align: middle;
text-align: center;
}