code: public final boolean contains(final AbstractHtml tag)
              throws NullValueException, NotRenderedException {
  
          if (tag == null) {
              throw new NullValueException(
                      "tag object in browserPage.contains(AbstractHtml tag) method cannot be null");
          }
  
          if (tagByWffId == null) {
              throw new NotRenderedException(
                      "Could not check its existance. Make sure that you have called browserPage#toHtmlString method atleast once in the life time.");
          }
  
          final DataWffId dataWffId = tag.getDataWffId();
          if (dataWffId == null) {
              return false;
          }
          return tag.equals(tagByWffId.get(dataWffId.getValue()));
      }
docs: To check if the given tag exists in the UI. <br>
  NB:- This method is valid only if {@code browserPage#toHtmlString} or
  {@code browserPage#toOutputStream} is called at least once in the life
  time.
  
  @param tag
  the tag object to be checked.
  @return true if the given tag contains in the BrowserPage i.e. UI. false
  if the given tag was removed or was not already added in the UI.
  @throws NullValueException
  throws this exception if the given tag
  is null.
  @throws NotRenderedException
  if the {@code BrowserPage} object is not
  rendered. i.e. if
  {@code browserPage#toHtmlString} or
  {@code browserPage#toOutputStream} was
  NOT called at least once in the life
  time.
  @since 2.1.7
  @author WFF