基础代码：

<div class="box"><div class="content"></div>
</div>

css实现居中的几种方式：

        1、flex布局（水平垂直）

.box {width: 200px;height: 200px;background-color: pink;display: flex;justify-content: center;align-items: center;
}
.content {width: 100px;height: 100px;background-color: lawngreen;
}

        2、margin:auto;（水平）

.box {width: 200px;height: 200px;background-color: pink;
}
.content {width: 100px;height: 100px;background-color: lawngreen;margin: auto;
}

        3、固定定位+transform（水平垂直）

.box {width: 200px;height: 200px;background-color: pink;position: relative;
}
.content {width: 100px;height: 100px;background-color: lawngreen;position: absolute;left: 50%;top: 50%;transform: translate(-50%, -50%);
}

        4、vertical-align: middle;（垂直）

.box {width: 200px;height: 200px;background-color: pink;
}
.content {width: 100px;height: 100px;background-color: lawngreen;
}
.box::after, .content{display:inline-block;vertical-align:middle;
}
.box::after{content:'';height:100%;
}

        5、line-heihgt + text-align

.text-box {width: 100px;height: 50px;background-color: aquamarine;text-align:center;
}
.text-box span {line-height: 50px;
}<div class="text-box"><span>1</span></div>