On closer look, it appears that I got this wrong. The standard, as far as I can see, disallows lists as direct children of other lists, but MDN states the content of a list can be “zero or more <li> elements, eventually mixed with <ol> and <ul> elements.”, where I suppose “eventually” means “if your browser feels like it”. Browsers do seem to allow such structure. I’ve pushed this patch, which adds a somewhat ugly kludge to ‘normalize’ this kind of input.