It's true that void elements can't be self-closing in HTML, but the slash is still allowed. If you scroll down to 8.1.2.1 #6, you will see that the slash is jus

It's true that void elements can't be self-closing in HTML, but the slash is still allowed. If you scroll down to 8.1.2.1 #6, you will see that the slash is just ignored if present on void elements.