docs: @Nullable
    public String getStaticContent() {
      if (!hasValue()) {
        return null;
      }
      HtmlAttributeValueNode attrValue = (HtmlAttributeValueNode) getChild(1);
      if (attrValue.numChildren() == 0) {
        return "";
      }
      if (attrValue.numChildren() > 1) {
        return null;
      }
      StandaloneNode attrValueNode = attrValue.getChild(0);
      if (!(attrValueNode instanceof RawTextNode)) {
        return null;
      }
      return ((RawTextNode) attrValueNode).getRawText();
    }