On both macOS Sierra 10.12 and today’s El Capitan 10.11.5 (when Safari 10 is installed), Safari will support App Extensions built from a combination of JavaScript, CSS and native code written in Objective-C or Swift.
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More importantly, the new App Extensions architecture enables developers to distribute Safari Extensions as part of their app through the App Store.
A clear trend in this year’s WWDC was the number of different extension points and distribution channels Apple is opening up: iMessage Apps, Maps extensions, Safari extensions, third-party Siri, to name just a few. The Safari Extensions, in particular, are a new approach to distributing browser plugins compared to both Chrome and Firefox, as you can write compiled code for them!