To forestall confusion: If the smiley face on the right is not much much brighter than the page background (which is #ffffff), then your hardware does not support this and you are not seeing what others are seeing.
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Or you're using Safari because my hardware absolutely does support this (tested in Chrome and I am thankful that Safari does not support it because good grief.)
Safari absolutely will support HDR images if it doesn't already. It might not support this PNG hack, but it's inevitable that it'll support HDR HEVC or JPEG images since those are what's produced by iOS and Android cameras respectively, and they obviously aren't going to just ignore them.
iPhones have not captured HDR images until much, much more recently. No earlier than iPhone 12 at the soonest (when they first could capture HDR video), although they keep fiddling with which format they use for the result. iOS 17 was when they added support for displaying these images in UIKit & Swift, which was only like 2 years ago give or take. WWDC '23 was similarly when they started talking about handling HDR images. And they just recently announced they'll be adopting ISO 21496-1 at WWDC 2024. ISO 21496-1 being the gainmap style approach that Google & Adobe adopted with UltraHDR in 2023.