Unfortunately this post contains a lot of inaccuracies. For example, the claim that the WHATWG HTML Standard is future-facing and for incubation is false. Such additions are explicitly against the WHATWG working mode: see the sections on Additions and New Proposals.
It also misunderstands the outline algorithm, which isn't a requirement on browsers, but instead guidance for developers as to how to structure their pages. As such it's not some kind of speculative browser feature as this post characterizes it. (It's bad guidance for developers—but the proper fix for that isn't to include some warning box saying "this feature, which isn't meant for browsers anyway, isn't implemented in browsers" like the W3C HTML fork does. The proper fix is outlined in the WHATWG issue tracker: namely, creating better guidance for developers. Unfortunately, this is one of the 654 open issues that hasn't yet been fixed in master. We'd welcome your help!)
It's unfortunate that people are referring to this post as if it were revealing a deep difference between the W3C HTML fork and the WHATWG HTML Standard. Instead, it's just based on a series of misunderstandings. If people are interested in some more fact-based accounts of the differences, see Anne's blog post or compare the W3C fork GitHub activity with that of the WHATWG HTML Standard.