I just spotted a new link tag out in the wild on two sites, youtube.com, and maketecheasier.com. The tag looks like this: <link rel="canonical" href="/watch?v=U4Bk5rmIpic"/> on youtube's site. This tag acknowledges the fact that URLs no longer uniquely identify a resource. It could also have important implications for search engines, helping them to more easily process sites with multiple settings and ways of accessing pages and information.