What happens to child elements when you set opacity on a parent?
When setting opacity on a parent element, the effective opacity of child elements is multiplied by the parent's opacity. This means if a parent has opacity: 0.5 and a child has opacity: 0.5, the child's effective opacity will be 0.25. This is different from other CSS properties and can't be overridden by the child elements, making it important to consider when designing layered interfaces.