Interesting; just tried this on my M1 MacBook Air running 12.0.1, and Safari plays 4k HDR just fine (ignoring that it's not actually playing on an HDR-capable
display, but that's neither here nor there...).
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I do have a couple 'experimental' features enabled in the Develop menu that I'm not sure are enabled by default (
HDR Media Capabilities and
VP9 decoder) — you can enable the Develop menu using the checkbox at the bottom of the Advanced tab in Safari preferences ("Show Develop menu in menu bar"), then enable those features from the menu if they aren't already checked.
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