【前端芝士树】如何对元素块实现居中（垂直和水平方向都居中）？

<div class="container">
    <!--// Any content in here will be centered.-->
    <img src="fireworks.jpg" alt="fireworks">
</div>

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

.item{
    margin:0 auto;
    width:200px;
    height:200px;
}

.item{
    width:300px;
    height:200px;
    
    position:absolute;
    left:50%;
    top:50%;
    margin:-100px 0 0 -150px;
    //或者 margin: auto;
}

$(window).resize(function(){

    $('.item').css({
        position:'absolute',
        left: ($(window).width() - $('.item').outerWidth())/2,
        top: ($(window).height() - $('.item').outerHeight())/2
    });

});

// To initially run the function:
$(window).resize();

.item{
   position: fixed;
   margin: auto;
   top: 0;
   right: 0;
   left: 0;
   bottom: 0;

   width: 200px;
   height: 100px;
}

 .container {
    width: 400px;
    display: table-cell;
    vertical-align: middle;
}

.item {
    margin: 0 auto;
}