Source

Screen readers

Use screen reader utilities to hide elements on all devices except screen readers.

Hide an element to all devices except screen readers with .sr-only. Combine .sr-onlywith .sr-only-focusableto show the element again when it's focused (eg by a keyboard-only user). Can also be used as mixins.

Skip to main content 
<a class="sr-only sr-only-focusable" href="#content">Skip to main content</a>
// Usage as a mixin
.skip-navigation {
  @include sr-only;
  @include sr-only-focusable;
}