Why does CSS work with fake elements?

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

Example:

imsocool {
    color:blue;
}

<imsocool>HELLO</imsocool>

When my professor first saw me using this, he was a bit surprised that made-up elements worked and recommended I simply change all of my made up elements to paragraphs with ID’s.

Why doesn’t my professor want me to use made-up elements? They work effectively.

Also, why didn’t he know that made-up elements exist and work with CSS. Are they uncommon?