Safari will use the same WebExtensions API as Chrome/Firefox

Apple announced a new plan in an announcement for Apple developers. Developers can use the WebExtensions API to create web extensions in Safari 14 beta, thereby adding custom features to Safari.
The WebExtensions API is also an API used by other browsers such as Chrome, Firefox, and Edge. The new conversion tool provided by Xcode 12 Beta allows developers to port existing extensions from other browsers to Safari and will provide them in the Mac App Store this fall.
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The current mainstream browsers all use the WebExtensions API, and this extension of technology has become a de-facto standard. Apple using the same technology believes that it can better encourage developers to create new extensions for Safari or transplant existing extensions, thereby greatly enriching the extension ecology of Safari.

The Safari Web Extensions documentation page currently provides instructions for creating, developing, porting, and customizing extensions, as well as methods for evaluating extension compatibility. You can see the documentation for details.