/**
* Registers a CSS code block.
* @param string $css the content of the CSS code block to be registered
* @param array $options the HTML attributes for the `<style>`-tag.
* @param string $key the key that identifies the CSS code block. If null, it will use
* $css as the key. If two CSS code blocks are registered with the same key, the latter
* will overwrite the former.
*/
public function registerCss($css, $options = [], $key = null)
{
$key = $key ?: md5($css);
$this->css[$key] = Html::style($css, $options);
}