Users can choose to enable plug-ins for one session or every time they visit a specific website.
For frequently-visited websites, enabled plug-ins will remain active.
Take advantage of powerful new features, advanced developer tools, and cutting edge technologies in Safari to deliver the best-in class websites and apps.
you can choose as I do, to refresh web pages every visit to the page, or perhaps as you note above, change it to automatic. From IE - tools - internet options - general - temporary internet files - settings ..... I am talking about after setiing "automatic" under tools, internet options, general, automatic.
The default HTML5 video player in Safari automatically supports Picture in Picture.
And if you use a custom video player, it’s easy to add a Picture in Picture control using the Java Script presentation For the first time ever, you can sell and distribute Safari Extensions through the Mac App Store.
The future of extensions development takes place in Xcode, where you can bring new capabilities to your extensions using powerful native APIs and familiar web technologies.
Xcode makes it easy to create Safari Extensions using App Extension templates.