Read-only
About
The :read-only pseudo-class matches elements that are read-only, i.e. not editable.
Normal Input
Normal inputs don't match as they are not read-only.
Read-only Input
The selector matches inputs with the readonly attribute.
Disabled Input
The selector matches inputs with the disabled attribute too, because they are not editable.
Hidden Inputs
The selector matches inputs of type hidden too.
Buttons
The selector matches buttons, both enabled and disabled.
Content Editable
The selector matches other non-editable elements, like a div. If you make it editable, by adding thecontenteditable attribute, it won't match anymore.
Plain div
ContentEditable div