设置line-height属性
行内元素设置高度,并且文本垂直水平居中
span {background: red;line-height: 100px;align-items: center;display: flex;justify-content: center;}
span {background: red;line-height: 100px;align-items: center;display: inline-flex; /*宽度是文本宽度*/justify-content: center;}
span {background: red;line-height: 100px;text-align: center;display: block;}
span {background: red;line-height: 100px;text-align: center;display: inline-block;/*宽度是文本宽度*/}
span {background: red;line-height: 100px;text-align: center;/*position: relative;*//*position: fixed;*/position: absolute; /*会改变display为block,宽度是文本宽度*/}
span {background: red;line-height: 100px;text-align: center;/*float: right;*/float: left; /*会改变display为block,宽度是文本宽度*/}