flex 居中

            
            body {
    display: flex;
    height: 100vh;
}

div {
    margin: auto;
    background: green;
    width: 300px;
    height: 300px;
    display: flex;
    align-items: center;
    justify-content: center;

    p {
        width: 50px;
        height: 350px;
        background: pink;
    }
}