Safari also apply “HDR” to CSS filter: brightness() with value greater than 1. Instead of the over-exposure effect, this portion of the screen will be brighter.
You must have an HDR or EDR-capable screen and there must be an HDR video playing to activate the HDR context (can be <video> playing somewhere in a webpage).
Not to say for sure Apple Silicon chips support HW AV1 decode, but I would speculate that recent generations of their SoCs may have hardware support behind a software limitation through the lack of APIs.
Under /System/Library/Video/Plug-Ins/AppleVideoDecoder.bundle there's a Info.plist, inside there are references to AV1, below that is VTIsHardwareAccelerated = true.
AVD refers to Apple Silicon HW decoder. Side note, GVA means Intel, VCP means AMD.
I guess it will be like the situation of VP9. It was available at first via a special entitlement to YouTube iOS app [1] (com.apple.developer.coremedia.allow-alternate-video-decoder-selection) and later opened to all apps.