HDR works for the Photos app. Though it doesn’t work for social apps I’ve tried, I’ve seen similarly high brightness static images in Safari. If you can’t do it with canvas, I’m betting you can do it with a Blob object/blob: URL or even just a plain data: URL. You’ll also need to set the appropriate color space, otherwise it’ll default to sRGB. You very well may also need to use some non-web APIs (pick your binary interface of choice), if canvas clamps to 8-bit (which I’m too lazy to look up on mobile at this particular moment, but I’d be surprised if it doesn’t).