Kinda doubt they will achieve parity. Even if they do, for how long? The problem is that the spec is a moving target, you cannot justify maintaining a platform indefinitely while a complete version exist on the side.
I just got a built in to the browser pop-up add a few weeks ago from Mozilla about a VPN product... sheesh.
Nothing stops a new project (eg Brave) from implementing a new browser based on an existing engine. And you even have 3 big mature engines to choose from (gecko, website and blink).
The engine is responsible for the HTML, CSS and JavaScript support, which is the bit everyone seems concerned about re standards.
The wrapper around that engine is significant - and where most of the complaints actually seem to lie. Built in popups, tracking, phoning home etc.
I'm not sure we need another engine. By contrast there are quite a few browsers to choose from built on those engines.
Building a new -browser- is "easy". Building a new engine is "hard" (and would be very expensive to keep current.)
Incidentally 90% is likely useless. 99% -might- be enough, depending on which 1% is missing.