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

Why does CSS work with fake elements? Because it doesn't hurt anyone because you're not supposed to use them anyways.

Why doesn't my professor want me to use made-up elements? Because if that element is defined by a specification in the future your element will have an unpredictable behavior.

Also, why didn't he know that made-up elements exist and work with CSS. Are they uncommon? Because he, like most other web developers, understand that we shouldn't use things that might break randomly in the future.