More on protecting children

Through my experience of programming web pages and web services is seems apparent to me that every single web page contains html. This would probably be something obvious to most people. If you were to right click on the page you are on at any time and go to "view source" you can look at this html. One thing you might notice at the top of the page is the < meta > tag. This meta tag can be used to have keywords in it to be used by search engines or the name of the author of the page. It seems rather obvious to me that the government could easily go to the W3c, which basically sets the standards of html, and ask them to put some flag into the language that would be able to designate if the content of the page is legal to show to people under 18. Since a browser would be able to see this tag, it would be able to prevent itself from going to a site if it is restricted. This could be then government enforced. Maybe something like this already exists and I don't know about it, but it seems like common sense.