The add method of the CustomStateSet interface adds value representing a custom state to the CustomStateSet.
Custom elements with a specific state can be selected using the :state() pseudo-class, specifying the desired state as an argument.
add(value)valueA string that represents the custom state.
Undefined.
The following function adds the state checked to a CustomStateSet.
class MyCustomElement extends HTMLElement {
set checked(flag) {
if (flag) {
this._internals.states.add("checked");
}
}
}