framework/less/common/mixins/accessibility.less

// This mixin should **only** be used in this file. If you want to define your own
// custom outline style(s), override this mixin in your own theme extension, or in
// the Custom Less section of the Admin dashboard.
#private {
  .__focus-ring-styles() {
    // This uses the browser's default outline styles, rather than
    // using custom ones, which could introduce more issues

    // Source: https://css-tricks.com/copy-the-browsers-native-focus-styles
    outline: 5px auto Highlight;
    outline: 5px auto -webkit-focus-ring-color;
  }
}

/**
 * Adds a focus ring to an element.
 *
 * This is only shown when focus is provided via keyboard, using the
 * `:focus-visible` selector, and `:-moz-focusring` for older Firefox.
 */
.add-keyboard-focus-ring() {
  // We need to declare these separately, otherwise
  // browsers will ignore `:focus-visible` as they
  // don't understand `:-moz-focusring`

  // These are the keyboard-only versions of :focus
  &:-moz-focusring {
    #private.__focus-ring-styles();
  }

  &:focus-visible {
    #private.__focus-ring-styles();
  }
}

/** 
 * This mixin allows support for a custom focus
 * selector to be supplied.
 *
 * For example...
 * 
 *? button { .addKeyboardFocusRing(":focus-within") }
 * becomes
 *? button:focus-within { <styles>  }
 *
 * AND
 *
 *? button { .addKeyboardFocusRing(" :focus-within") }
 * becomes
 *? button :focus-within { <styles>  }
 */
.add-keyboard-focus-ring(@customFocusSelector) {
  @realFocusSelector: ~"@{customFocusSelector}";

  &@{realFocusSelector} {
    #private.__focus-ring-styles();
  }
}

/**
 * Allows an offset to be supplied for an a11y
 * outline.
 *
 * Useful for elements whose content is right up
 * against their bounds.
 *
 * `.addKeyboardFocusRingOffset(2px)` will add an
 *  offset of 2 pixels to the outline.
 */
.add-keyboard-focus-ring-offset(@offset) {
  .offset() {
    outline-offset: @offset;
  }

  &:-moz-focusring {
    .offset();
  }
  &:focus-visible {
    .offset();
  }
}

/**
 * Allows an offset to be supplied for an a11y
 * outline.
 *
 * Useful for elements whose content is right up
 * against their bounds.
 */
.add-keyboard-focus-ring-offset(@customSelector, @offset) {
  .offset() {
    outline-offset: @offset;
  }

  @realFocusSelector: ~"@{customFocusSelector}";

  &@{realFocusSelector} {
    .offset();
  }
}