public function addClass($className)
    {
        if(strlen($className) < 1)
        {
            throw new HTMLTagException("Class name is empty");
        }

        if(!$this->hasClass($className))
        {
            $classes = $this->getClasses() . " " . $this->removeMultipleSpaces(trim($className));
            $this->setAttribute("class",trim($classes));
        }

        return $this;
    }