code: protected boolean addToAttributeValueSet(final String value) {
          final Collection<AbstractHtml5SharedObject> sharedObjects = getSharedObjects();
          boolean listenerInvoked = false;
          final Collection<WriteLock> writeLocks = lockAndGetWriteLocks();
          try {
  
              final boolean added = getAttributeValueSet().add(value);
  
              if (added) {
                  setModified(true);
  
                  invokeValueChangeListeners(sharedObjects);
                  listenerInvoked = true;
  
              }
  
          } finally {
              for (final Lock lock : writeLocks) {
                  lock.unlock();
              }
          }
          pushQueues(sharedObjects, listenerInvoked);
          return listenerInvoked;
      }
docs: adds to the attribute value set.
  
  @param value
  @since 1.0.0
  @author WFF
  @return