Where in the web standards do you find specifications like how bookmark toolbars and menus should be organized, or that Shift-Ctrl-T should bring back a tab that was closed?
We're talking about a browser _engine_. The shell/chrome is irrelevant in this context
But some parts are far from being trivial engineering and are not part of any specs. Just consider the security infrastructure (multiprocess, sandboxing etc.) and how that impacts the overall design of the engine itself.