Sure, for a well built standards compliant website making appropriate use of HTML elements. There are a lot of websites that do their own thing, use lots of custom Javascript, where standard browser features like tabbing through their elements don't work, and they don't work well with existing accessibility tools. I'm skeptical that an OS or browser level feature is going to be able to make sense of them. That's their fault, but it's the reality of a huge part of the web.