Despite a fair bit of debugging I haven't figured out why the test color swatches on the wide-gamut website are decoding to NV12, the decoder refuses to output P010 even if I override the subtype, so they don't end up getting the RGB10A2 format with BT2100 PQ colorspace as would be needed for correct HDR display. It is possible these are legitimately 8bit videos in a P3-extended space of some kind but it isn't clear what is going on there.

Note that the color swatches on that page (https://www.wide-gamut.com/test) are in fact videos, not images, the page has too many videos on one page to display them as HDR overlays (https://bugzilla.mozilla.org/show_bug.cgi?id=1998096) but that isn't the issue, in isolation they still don't show as HDR.