Jim, a V-chip in the TV world works because of the way that the encoding is made. The data for the V-chip encoding is done in a similar manner to close captions or XDS encoded data. The information is broadcast on a continuous basis, not just at a single time at the start of the show (the little logo you see at the start of the show is only there to show you, technically it does nothing.
http://www.linkelectronics.com/htm/techvchp.htm
Web pages are served in a different manner, one shot there you go, all the data is in one place. Making the browser (which is the software normally used for reading web pages) check for a simple meta tag is a piece of cake job, a very few lines of code and you are off to the races.
The government needs to pass simple rules: All web pages must be rated (General to Restricted / adult / XXX ) and all browsers must support a simple system to read these tags and block unwanted websites.
Done.
Sadly, the simple answers will never be simple.
Alex