Sure, technical documents are long but that still doesn’t support the original claim that they are “unnecessarily complex, bloated, convoluted” and it’s actually evidence against the assertion that they’re “difficult to implement without specific knowledge of its features”: most of why those are long documents is that they carefully detail how necessarily complex systems interact in sufficient detail to implement them whereas the Office XML specs at least historically had things like flags telling to behave like, say, Word95 without fully specifying the behaviour in question.

Sure the spec might be enormous but you don't need to touch it at all to be productive quickly. In no HTML or CSS tutorial i'v ever seen was a reference to the spec nor did i need to go there to solve something. And that in itself is another proof how nicely it is designed actually. Because on the other hand there are other document types or schemas where you absolutely have to go to the spec because it's is so cryptic and badly designed and not self-explaining that there is nothing else you can do.