#Composition is the Goal
I see several discussions now that vacillate around on encapsulation qualities of Shadow DOM and the flavors thereof. It's all my fault. I stuck encapsulation into the problem statement of the spec. I even glued the somewhat irrelevant term functional encapsulation on top of it. What a dork.
It was a couple of years ago, when one of my colleagues read the spec and noted: "Dude, it's like your story and whatever, but the way you tell it, Shadow DOM is not about encapsulation. It's about composition". The world went blank around me. I was blinded by insight.
My hipster colleague was right, of course. Encapsulation is just a tool. Composition is the goal.
This is why the people who'd never used Shadow DOM in real life get really hung up on the details of encapsulation, while those who actually use Shadow DOM stare at them quizzically.