docs: public String attr(String attributeKey) {
          Validate.notNull(attributeKey);
          if (!hasAttributes())
              return EmptyString;
  
          String val = attributes().getIgnoreCase(attributeKey);
          if (val.length() > 0)
              return val;
          else if (attributeKey.startsWith("abs:"))
              return absUrl(attributeKey.substring("abs:".length()));
          else return "";
      }