The spec does not explicitly encourage using will-change on an element that is currently invisible. For example:

.child {
    display: none;
    transition: translate 2s;
}

.parent:focus > .child {
    display: block;
    translate: 100px;
    @starting-style {
        translate: initial;
    }
}

.parent:hover > .child {
    will-change: translate;
}

I recommend the spec add use cases like this. This would imply that the browsers likely have taken will-change into consideration even if the element is currently invisible.