src/Assets.php
public function getExternalCss($key = null)
    {
        if (isset($this->externalCss[$this->key($key)])) {
            $externalCss = $this->externalCss[$this->key($key)];
            return $this->getCssTags($externalCss);
        }

        return null;
    }
public function getExternalCss($key = null)
    {
        if (isset($this->externalCss[$this->key($key)])) {
            $externalCss = $this->externalCss[$this->key($key)];
            return $this->getCssTags($externalCss);
        }

        return null;
    }
@param null $key

@return string