> they've been working on it.

Could you provide a link to any recent communication from them about this, or are you just speculating based on that other platforms support it?

I don't know about a communication... but I can do one better! You can download the latest Safari Technology Preview on macOS, toggle the "Support HDR Display" feature flag, and see it working (for certain image formats and HDR encodings - it's a flag in preview for a reason). E.g. the images at https://ccameron-chromium.github.io/hdr-jpeg/index.html will be brighter (if your mac has an HDR display attached with HDR enabled) but not the images from this HN post yet.