I wouldn't say it's a "new" syntax as much as it's an experimental syntax (at least according to the Apple dev stating the reason for the choice in the pre-rele

I wouldn't say it's a "new" syntax as much as it's an experimental syntax (at least according to the Apple dev stating the reason for the choice in the pre-release version and explaining that they're not committed to the new syntax for the release version [https://lists.w3.org/Archives/Public/public-whatwg-archive/2015Jun/0020.html] https://lists.w3.org/Archives/Public/public-whatwg-archive/2...). I'd be surprised if it makes it into Safari's released version, and will hold my outrage until then.