head.html

<script>
class HttpError extends Error {
  constructor(response) {
    super(`${response.status} for ${response.url}`);
    this.name = 'HttpError';
    this.response = response;
  }
}

function loadJson(url) {
  return fetch(url)
    .then(response => {
      if (response.status == 200) {
        return response.json();
      } else {
        throw new HttpError(response);
      }
    })
}
</script>

<style>
.promise-avatar-example {
  border-radius: 50%;
  position: fixed;
  left: 10px;
  top: 10px;
}
</style>