Testing font awesome icons as svg sprites

Implemented in accordance to
https://fontawesome.com/how-to-use/on-the-web/advanced/svg-sprites (or maybe not? What I'm missing?

Hover the black area and you should see a title, which I did not add

The code is pretty simple:

Environment: Chrome 70.x, Firefox 63.x on Ubuntu 16.x

And the question is...