mastodon/app/javascript/styles/_mixins.scss

@mixin avatar-radius() {
  border-radius: 4px;
  background: transparent no-repeat;
  background-position: 50%;
  background-clip: padding-box;
}

@mixin avatar-size($size:48px) {
  width: $size;
  height: $size;
  background-size: $size $size;
}