public function addTag(string $tagName, string $src)
    {
        if (preg_match('/^[a-z\-]+$/', $tagName) !== 1) {
            throw new \InvalidArgumentException('The tag name provided is not valid! It may contain letters (a-z) and dashes (-).');
        }
        $this->tags[strtolower(trim($tagName))] = $src;
    }