Interestingly, the DCI-P3 demo is working in Chrome on Mac now. (It didn't used to-- Safari/Webkit was the only browser that handled DCI-P3 images on Mac for a while.)
That's good. I assume you mean the webkit logo test. I wonder what's different about color management on macOS such that this was hard for the other browsers to support.
Correctly decoding images tagged in any color space has worked correctly in both Firefox and Chromium for as long as I can remember on Linux. In fact, it's even working "correctly" now on Chromium+Linux, it's just that they're seemingly choosing to render everything to an intermediate color space of sRGB before rendering out to the screen.
I found a good additional test to illustrate the problem with Chrome. [1] With the first test, "How far from sRGB is your display color gamut" the bars look exactly the same in Chrome to me. [2] But they're not oversaturated (i.e. in the monitor color space), they're seemingly correct sRGB colors, as displayed in a color managed way on my screen. The ProPhoto tagged image in Chrome looks exactly like the sRGB tagged image does in Firefox. So Chrome is performing color management, it's just effectively clamping the output to the sRGB gamut.
Safari uses an OS level thing for decoding images — this is why Safari 14 for MacOS “Big Sur” can display WebP images, but Safari 14 Catalina cannot. So it is possible that Chrome is using the same system lib to render images, which thusly can display P3.
But I would hazard a guess that if you tried to pump P3 color through CSS or an SVG in Chrome, that would fail.
