* It mixes concerns of HTML and CSS - e.g. <pre> eating newlines is a result of the HTML parser, but whitespace collapsing is specified in CSS.
* It suggests turning HTML from a markup language into... whatever incompatible thing the author came up with. Arguably it's not much worse than XHTML, but I don't expect better adoption.
* The final suggestion is to add a character reference to CSS - the sole issue being that CSS does not see character references, those are turned by HTML into Unicode codepoints. Also, the set of character references is closed, for good reasons.[0]
* (Nit, but "block formatting context" does not mean what the author thinks it means. Flex items behave like blocks because they become blockified - BFCs solve a separate, similarly hairy issue (floats & margin collapsing.))
[0]: https://github.com/whatwg/html/blob/main/FAQ.md#html-should-...