What's wrong with my CSS component?

I have this code in a plugin. The only function inside the original CSS class is tag so I tried to return something else.

class TestClass extends Kirby\Component\CSS {
  public function tag($url, $media = null) {
  	return 'Test';

$kirby->set('component', 'CSS', 'TestClass');

I get no errors, but the CSS output in my template is still the same CSS file html. It seems like nothing happends. I expected Test as output or at least an error.

From the docs

A component can be registered in a plugin file with the kirby()->set() method:
$kirby->set(‘component’, ‘thumb’, ‘MyThumbClass’);
With the second argument, you specify which of the components you want to replace. The classname of your custom component class must be passed as last argument. The class must be loadable or exist.
Your custom class must extend the default component class!

What did I miss?

It should be css instead of CSS I think. I guess I need to lowercase everything when registering components, so this cannot lead to an error.

Yes, that works. I was confused by the original class name which is “CSS”.

Lowercase or throw an error would be fine I think.