css/modules/primer-avatars/lib/avatar-stack.scss

// Stacked avatars can be used to show who is participating in thread when
// there is limited space available.
//
// No styleguide references
.avatar-stack {
  display: inline-block;
  white-space: nowrap;

  .avatar {
    position: relative;
    z-index: 2;
    display: inline-block;
    width: 20px;
    height: 20px;
    box-sizing: content-box;
    margin-right: -15px;
    background-color: $bg-white;
    border-right: 1px solid $white;
    border-radius: 2px;
    transition: margin 0.1s ease-in-out;

    &:only-child {
      background-color: transparent;
    }

    &:first-child {
      z-index: 3;
    }

    &:last-child {
      z-index: 1;
      margin-right: 0;
      border-right: 0;
    }
  }

  // When more than one avatar, margin-left will animate from -15 to 2, and reveal the stack
  &:hover .avatar {
    margin-right: 3px;

    &:last-child {
      margin-right: 0;
    }
  }
}