src/_core/_mixins/_tab-focus.scss
// WebKit-style focus
@mixin tab-focus() {
// WebKit-specific. Other browsers will keep their default outline style.
// (Initially tried to also force default via `outline: initial`,
// but that seems to erroneously remove the outline in Firefox altogether.)
outline: 5px auto -webkit-focus-ring-color;
outline-offset: -2px;
}