Why does CSS work with fake elements?

Question

In my class, I was playing around and found out that CSS works with made-up elements.

Example:

Answer 1

Actually you can use custom elements. Here is the W3C spec on this subject:

http://w3c.github.io/webcomponents/spec/custom/

And here is a tutorial explaining how to use them:

http://www.html5rocks.com/en/tutorials/webcomponents/customelements/

As pointed out by @Quentin: this is a draft specification in the early days of development, and that it imposes restrictions on what the element names can be.