What are the CSS classes for block instances?
After I created a second instance for a block, I saw that Drupal 8 doesn't create a class to mark it, but rather creates a similar, yet almost identical CSS id (#) name for it which is not zero-based index in this case, rather one-based-indexed.

For example, if the first instance' id is #block-contact-form, the second will be #block-contact-form-2.

If we want to hide only the first instance, we can now do:

media screen and (min-width: 768px) {

#block-contact-form {display: none}

}


If we wanted to hide the second one we would change #block-contact-form to #block-contact-form-2.