docs: protected static String getTagOpen(
          String tagName,
          String containerName,
          String tagClass,
          boolean nested,
          boolean online,
          String containerData) {
  
          StringBuffer buffer = new StringBuffer(32);
          buffer.append("<").append(tagName).append(" ");
          if (online && nested) {
              // omit generated ids when online
          } else {
              buffer.append(" id=\"").append(containerName).append("\" ");
          }
          if (containerData != null) {
              buffer.append(" " + CmsGwtConstants.ATTR_DATA_CONTAINER + "=\"").append(containerData).append("\" ");
              // set the marker CSS class
              tagClass = tagClass == null
              ? CmsContainerElement.CLASS_CONTAINER
              : tagClass + " " + CmsContainerElement.CLASS_CONTAINER;
          }
          if (CmsStringUtil.isNotEmptyOrWhitespaceOnly(tagClass)) {
              buffer.append("class=\"").append(tagClass).append("\" ");
          }
          buffer.append(">");
          return buffer.toString();
      }