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 <a href="https://github.com/whatwg/html/issues/83#issuecomment