wagtail/client/scss/components/_skiplink.scss

.skiplink {
  display: block;
  position: fixed;
  top: -1000rem;
  inset-inline-start: 1rem;
  z-index: 3000;

  &:focus {
    top: 1rem;
  }

  &.button {
    background: theme('colors.surface-button-default');
    border: theme('colors.surface-button-default');
  }
}