And we need a standard where websites can self-rate their own content. Then locked devices can just block all content that isn't rated "G" or whatever.

Wrong incentive. If you don’t give a shit about exposing children to snuff or porn, but do give a shit about page views and ad revenue, you obviously don’t rate your content or rate it as G to increase that revenue.

I imagine there would be a set of filters, including some on by default that most adults keep for themselves. For example, most people don't want to see gore. More would be OK with sexual content, even more would be OK with swear words, ...