code: public boolean exists(final AbstractAttribute attribute)
              throws NullValueException {
  
          if (attribute == null) {
              throw new NullValueException("attribute cannot be null");
          }
  
          for (final AbstractHtml ownerTag : attribute.getOwnerTags()) {
              if (browserPage.contains(ownerTag)) {
                  return true;
              }
          }
  
          return false;
      }
docs: Checks the existence of an attribute instance.
  
  @param attribute
  @return true if the given attribute instance exists anywhere in the
  browser page.
  @throws NullValueException
  if the {@code attribute} is null
  @since 2.1.8
  @author WFF