Based only on the majority opinion. This will require some amount of judgment, but the idea is to capture the core/primary/central reason(s), not everything which is discussed.
The core of the decision is pretty clearly that section 13 is not self-executing. (Which, as @Najawin says, is a joke...but that's not relevant to resolving this.)
Okay so as far as I understand it, the majority opinio can be summarized as thus:
States don't have the authority to decide on the application of this clause
The power to decide on the application rests with congress
That's it, there are no more conclusions.
Since power rests with congress, I believe it means it's not self-executing. I also think that saying that states don't have the authority means the Colorado court didn't have standing, and other should also resolve as true, but I am not a lawyer so maybe I misunderstood something.
@Shump Standing means that Trump's team wouldn't have the ability to bring the case. This is self executing.
(Which, you know, is a joke, because if you read article 3 it's clearly, obviously, self executing. But it was always going to be the way they tossed this case.)
@MattLashofSullivan ideally, the Marks Rule, as that seems to best capture the spirit of the market, acknowledging that that might be hard to determine. If there is no clear common denominator or narrowest opinion...that's a hard one. My first instinct is to treat every concurring vote as a vote for the grounds the concurrence relies on, and see which grounds reach the most votes, and mark as yes all the tied-for-top-vote-getters. But I'm open to arguments (soon!) that a different approach would be better.
@josharian On further thought, that’s too complicated. Decision: Marks Rule if possible, plurality if not.