box width 100; margin 0 auto; top 0; left 0; background-color #fff; head 开始 head height 100px; display flex; justify-content space-between; align-items center; position fixed; head ul heada float

可以给head导航栏的父元素设置position: fixed;和top: 0;，这样就可以让导航栏固定在页面的顶部不随页面滑动而改变了。例如：

.head { height: 100px; display: flex; justify-content: space-between; align-items: center; position: fixed; top: 0; width: 100%; background-color: #fff; }