Hmm, that would work too. Under this kind of system, the laws protecting children wouldn't ban serving possibly-adult content, or require age checking, but would mandate that you must label certain content with the correct and appropriate age label.
Lack of label should be equated to UNRATED, just like for movies. By _default_ the Internet should be for Adults who have their own right to choose what they see or don't see.
Let the OS / browser / etc WHITELIST things based on a claim something is safe for children, or rated in an academic context (E.G. Wikipedia / medical / etc).