grid-span在没有断点的情况下无法工作

Question

在下面的代码示例中，我试图在移动设备上将.l-branding div的宽度定义为5个网格，并在断点$ tab上将宽度更改为3个网格。在断点处，它可以按照需要工作，但对于移动设备，网格跨度根本不会发生任何变化。类似于.l-region - navigation和.l-region - header

我做错了吗？

  .l-branding {
    @include grid-span(5, 1); //mobile
    background:url(../../../images/bg-footer-credits.png) center center;
    text-align:center;
    @include breakpoint($tab){
    @include grid-span(3, 1);
    height:104px;
    }  
  }
 // end l-branding 

  .l-region--navigation {
  @include grid-span(7, 6); //mobile
  @include breakpoint($tab){
    @include grid-span(6, 4);
    }
  }
  .l-region--header {
  @include grid-span(12, 1); //mobile
  clear:both;
  @include breakpoint($tab){
    @include grid-span(3, 10);
    clear:none;
    }
  }