Acquisitions work the same as they always have. Roughly, if half the voting shareholders approve of the deal it will go through.

Edit: from the first result in Google:

> Casalena and long-term investors General Atlantic and Accel, which make up about 90% of the Squarespace’s voting shares, have agreed to vote in favor of the transaction.

So I guess the shareholders mentioned in the subheading of the linked press release own a majority.