static get annotationStylePromise() {
    const styles = [VIEWER_CSS, "./annotation_layer_builder_overrides.css"];
    return shadow(this, "annotationStylePromise", loadStyles(styles));
  }
For the reference tests, the full content of the various layers must be
visible. To achieve this, we load the common styles as used by the viewer
and extend them with a set of overrides to make all elements visible.

Note that we cannot simply use `@import` to import the common styles in
the overrides file because the browser does not resolve that when the
styles are inserted via XHR. Therefore, we load and combine them here.