If some old IE version did things a certain way, even the most modern browser will want to do things in a similar way to remain compatible. Therefore, the standards bodies will try to reverse-engineer the existing behaviors and then create standards based on those. That way, modern code can simply follow the spec and remain compatible.
The HTML5 parsing algorithm is an example of this. Old browsers tried to "fix" broken HTML by guessing where things like missing closing tags were supposed to go. The HTML 4 specification never described this logic, yet it was there in the wild. The new HTML 5 specification made a point of reverse-engineering the repair algorithms and actually documenting them, so now everyone can be compatible going forward, both with each other and with legacy. Just follow the spec.
If some old IE version did things a certain way, even the most modern browser will want to do things in a similar way to remain compatible. Therefore, the standards bodies will try to reverse-engineer the existing behaviors and then create standards based on those. That way, modern code can simply follow the spec and remain compatible.
The HTML5 parsing algorithm is an example of this. Old browsers tried to "fix" broken HTML by guessing where things like missing closing tags were supposed to go. The HTML 4 specification never described this logic, yet it was there in the wild. The new HTML 5 specification made a point of reverse-engineering the repair algorithms and actually documenting them, so now everyone can be compatible going forward, both with each other and with legacy. Just follow the spec.