Great, so if I build a website based purely on the WHATWG specs, it will work in all browsers, correctly? No, it won't. I can take the A4 paper spec and build

Great, so if I build a website based purely on the WHATWG specs, it will work in all browsers, correctly? No, it won't. I can take the A4 paper spec and build a printer that takes that paper, and I know paper will comply with it. And the other way around. You can't build a website just from WHATWG specs, and you can't, excluding the parts about backwards compatible parsing, easily build a new browser from scratch either. A standard is an a-priori written document that describes the entire API surface, so that people on both sides can develop based on the standard without having to verify with actual implementations. The WHATWG documents are useless for this purpose.