It's not. How HTML is rendered was literally not defined between browsers until mid-00s. Hence the difference in the boxing model, for example.
Bt yes, besides HTML there are hundreds of other specs, and those may or may not be implemented across browsers.
> The underspecified natural language specs lead to subtly different behaviors in different browsers.
You keep saying this, and I don't think you even understand what you're saying.
> Full specs of a complex systems are very complex.
Yes, they are. What you keep proposing though, is to reverse engineer behaviour from millions of lines of code. I'm still waiting for an answer to my question about datalistoptions test.
The technical blueprints are as important than the natural language notes in the blueprints. In software, code is blueprint.
Is this sarcasm? Writing web apps that work the same across browsers is still the most painful part of web development. The underspecified natural language specs lead to subtly different behaviors in different browsers.
Full specs of a complex systems are very complex.