That is true, but it is much easier to accurately implement a VM spec correctly than complex high-level specs like CSS and HTML, which leave a lot of ambiguity. New hardware interfaces are also a lot easier to specify when you're talking about functions which are rarely going to be called directly by human programmers, since they will usually be accessed through higher-level frameworks.

Overall, the goal is to make implementing a browser a much simpler and direct process, and to defer as much complexity as possible to libraries which can be easily downloaded and updated. This would not only result in more a heterogeneous ecosystem for web developers, but fewer browser bugs and incompatibilities.