code: public boolean markAsImportant(final String styleName) {
          if (styleName == null) {
              return false;
          }
          final long stamp = lock.writeLock();
          try {
  
              final String trimmedStyleName = StringUtil.strip(styleName);
              final String value = super.getAttributeValueMap()
                      .get(trimmedStyleName);
              if (value != null && !value.isEmpty()) {
                  return super.addToAttributeValueMap(trimmedStyleName,
                          value + " " + IMPORTANT);
              }
              return false;
          } finally {
              lock.unlockWrite(stamp);
          }
      }
docs: marks as important. i.e. it will add {@code !important} from the
  corresponding styleName value
  
  @param styleName
  @return true if it's marked as important or false if it already marked as
  important
  @since 1.0.0
  @author WFF