wagtail/client/scss/objects/_avatar.scss

// user avatars
.avatar {
  border-radius: 100%;
  position: relative;
  display: inline-block;
  vertical-align: middle;
  text-align: center;
  overflow: hidden;
  width: 50px;
  height: 50px;

  img {
    position: absolute;
    z-index: 2;
    top: 0;
    // Remove once we drop support for Safari 13.
    // stylelint-disable-next-line property-disallowed-list
    left: 0;
    inset-inline-start: 0;
    // Remove once we drop support for Safari 13.
    // stylelint-disable-next-line property-disallowed-list
    right: 0;
    inset-inline-end: 0;
    border: 0;
  }

  &.small {
    vertical-align: middle;
    margin: 0 0.5em;
    width: 25px;
    height: 25px;
  }

  &.large {
    width: 100px;
    height: 100px;
  }

  &.square {
    border-radius: 0;

    &:before {
      border-radius: 0;
    }
  }
}