Does safari support html5 video from Youtube

Safari supports HTML5.

If YouTube video doesn't play, try either disabling or uninstalling extensions like ClickToFlash.

Safari > Preferences > Extensions

If Safari is slow, stops responding, quits unexpectedly, or has other issues

https://support.apple.com/en-us/HT203353

Section: Manage Extensions

https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac

Related: HTML5 test. https://html5test.com/index.html

More nuance is needed: The streaming protocol for YT 4K (and higher) is still HMTL5, but it uses WebM container format with VP9 video codec, and that isn’t supported in Safari (and/or QuickTime).

Let’s hope this silliness ends with the roll-out of AV1.

Yes. Years before YouTube dropped Flash in favor of HTML5 video.

Just not 4K HTML5 streaming video, as Google has gagged that feature in preference of their own proprietary streaming protocol that does support 4K.