scss/helpers/_visually-hidden.scss - external/github.com/twbs/bootstrap - Git at Google

 //
 // Visually hidden
 //

 .visually-hidden,
 .visually-hidden-focusable:not(:focus):not(:focus-within) {
   @include visually-hidden();
 }
	//
	// Visually hidden
	//

	.visually-hidden,
	.visually-hidden-focusable:not(:focus):not(:focus-within) {
	@include visually-hidden();
	}