Apple today released a new update for Safari Technology Preview, the experimental browser Apple first introduced two years ago in March of 2016. Apple designed the Safari Technology Preview to test features that may be introduced into future release versions of Safari.
The Safari Technology Preview update is available through the Software Update mechanism in the Mac App Store to anyone who has downloaded the browser. Full release notes for the update are available on the Safari Technology Preview website.
Apple's aim with Safari Technology Preview is to gather feedback from developers and users on its browser development process. Safari Technology Preview can run side-by-side with the existing Safari browser and while designed for developers, it does not require a developer account to download.
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If you submit a 4K file to YouTube that's already H.26x, they transcode it to their format and delete the original.
It's a dick move.
EDIT: if you are a casual user it may be ok.