The obvious problem with this
Is that when a movie is released its content is then largely static. Nobody can suddenly insert a sex scene into Star Wars: The Phantom Menace dvd that my nephew owns. Web-sites change. Frequently. Do you have to submit the site for frequent re-classification?
And what do you do about user-content-driven sites? Twitter, Facebook, YouTube et al have all sorts of dubious content/views on them Do you rank them as 18 and over?
And do they really expect kids to self-certify? And do they really expect the majority of clueless parents to know how to block this stuff (and stop their kids from secretly unblocking it?)?