Firefox 99.0 (Linux) here, the square is invisible.

> which makes no sense to me since even if the monitor colour calibration was wrong, I'd except it to apply to both the page and the image equally.

Color mgmt by browsers is, when it is applied, only applied to images (neither videos nor "CSS colors", broadly speaking). I'd bet you have an ICC profile associated with your monitor.

On Linux, I can see the square in Firefox, but not in Chrome. Interestingly enough, the color in Chrome is identical to the color of the square in Firefox, but when I force sRGB in Chrome (chrome://flags/#force-color-profile), it becomes identical to the background color in Firefox. I don't know what to make of that