@media

not :否定媒体查询，必须制定媒体类型

only

and :就是全部满足菜执行

，：有一个满足就成立，等价于or

如果文档宽度小于 300 像素则修改背景颜色(background-color):

@media screen and (max-width: 300px) {??? body {
	???????
	background-color:lightblue;??? }}

?如果像素小于700,怎隐藏字段

<!-- 移动端 -->
<style scoped>
@media only screen and (max-width: 700px) {
    /* .clearfix::after{
        content: '';
        display: block;
        clear: both;
    } */
    .head-logo {
        width: 50px !important;
        height: 50px !important;
        margin-left: 30px !important;
    }
    .logo {
        display: block !important;
        width: 180px;
        height: 50px;
        float: right;
        margin-top: 10px !important;
        margin-right: 80px !important;
    }
    #student .list {
        display: block !important;
    }
    .list li:last-of-type {
        padding: 0 10px !important;
        margin: 0 10px !important;
    }
    .msg {
        z-index: 999;
    }
    .practice{
        display: none;
    }
}
</style>