Specifically, this is the document outline algorithm. But it’s not so much “not always implemented properly” as “not implemented at all”. No user agent has impl

Specifically, this is the document outline algorithm. But it’s not so much “not always implemented properly” as “not implemented at all”. No user agent has implemented it or seems likely to, and I don’t know of anything other than conformance checkers that implements it. https://github.com/whatwg/html/issues/83 https://github.com/whatwg/html/issues/83 shows a bit of the discussion that’s gone on; whether to remove it, simplify it drastically to the point that it’s actually sanely implementable, &c.