响应式

 

    nav{width: 100%;height: 50px;background: black}

    ul{list-style: none;background: lightblue;height:100%;

    margin: 0 auto;display: flex;justify-content: space-between}

    @media screen and (min-width: 1200px){

      ul{width: 1200px}

    }

    @media screen and (min-width: 767px) and (max-width: 1200px){

      ul{width: 100%}

    }

    @media screen and (max-width: 767px){

      ul{display: none}

    }