I've had one foot stuck in this #DCI-P3 #ColorSpace rabbit hole for days. Found a simple test page for browser support: https://www.wide-gamut.com/test
But even in iOS Safari it did not show #P3 support... Last night I found out why - my #DarkReader extension blocked it. Disable DarkReader and the differences from #sRGB are obvious. (Safari's native Reader Mode allows P3, but it breaks a bit of the page content.) Other iOS browsers also support P3. No Windows or Linux browsers I tried can.