transform垂直居中

html

<div class="demo"></div>

css

div {
    width: 100px;
    height: 100px;
    position: fixed;
    left: 50%;
    top: 50%;
}

.demo {
    border: 1px solid #ddd;
    background-color: #ddd;
    transform: translate(-50%, -50%);
}